Internet DRAFT - draft-kompella-larp
draft-kompella-larp
MPLS WG K. Kompella
Internet-Draft R. Balaji
Intended status: Standards Track Juniper Networks, Inc.
Expires: August 18, 2014 February 14, 2014
Label Distribution Using ARP
draft-kompella-larp-00
Abstract
This document describes extensions to the Address Resolution Protocol
to distribute MPLS labels for IP host addresses. Distribution of
labels via ARP enables simple plug-and-play operation of MPLS, which
is among the key goals of "MPLS Fabric" architecture.
Requirements Language
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 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 18, 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Kompella & Balaji Expires August 18, 2014 [Page 1]
Internet-Draft Label Distribution Using ARP February 2014
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Approach . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview of Ethernet ARP . . . . . . . . . . . . . . . . . . 3
3. L-ARP Protocol Operation . . . . . . . . . . . . . . . . . . 4
3.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 4
3.2. Asynchronous operation . . . . . . . . . . . . . . . . . 5
3.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
4. For Future Study . . . . . . . . . . . . . . . . . . . . . . 5
5. L-ARP Message Format . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes extensions to the Address Resolution Protocol
(ARP) [RFC0826] to advertise label bindings for IP host addresses.
While there are well-established protocols, such as LDP, RSVP and
BGP, that provide robust mechanisms for label distribution, these
protocols tend to be relatively complex, and often require detailed
configuration for proper operation. There are situations where a
simpler protocol may be more suitable from an operational standpoint.
An example is where an MPLS Fabric is the underlay technology in a
Data Centre; here, MPLS tunnels originate from host machines. The
host thus needs a mechanism to acquire label bindings to participate
in the MPLS Fabric, but in a simple, plug-and-play manner. Existing
signaling/routing protocols do not always meet this need. Labeled
ARP (L-ARP) is a proposal to fill that gap.
[TODO-MPLS-FABRIC] describes the motivation for using MPLS as the
fabric technology.
1.1. Approach
ARP is a nearly ubiquitous protocol; every device with an Ethernet
interface, from hand-helds to servers, have an implementation of ARP.
ARP is plug-and-play; ARP clients do not need configuration to use
ARP. That suggests that ARP may be a good fit for devices that want
Kompella & Balaji Expires August 18, 2014 [Page 2]
Internet-Draft Label Distribution Using ARP February 2014
to source and sink MPLS tunnels, but do so in a zero-config, plug-
and-play manner, with minimal impact to their code.
The approach taken here is to create a minor variant of the ARP
protocol, labeled ARP (L-ARP), which is distinguished by a new
hardware type, MPLS-over-Ethernet. Regular (Ethernet) ARP (E-ARP)
and L-ARP can coexist; a device, as an ARP client, can choose to send
out an E-ARP or an L-ARP request, depending on whether it needs
Ethernet or MPLS connectivity. Another device may choose to function
as an E-ARP server and/or an L-ARP server, depending on its ability
to provide an IP-to-Ethernet and/or IP-to-MPLS mapping.
2. Overview of Ethernet ARP
In the most straightforward mode of operation [RFC0826], ARP queries
are sent to resolve "directly connected" IP addresses. The ARP query
is broadcast, with the target-protocol-address field carrying the IP
address of another node in the same subnet. All the nodes in the LAN
receive this ARP query. All the nodes, except the node that owns the
IP address, ignore the ARP query. The IP address owner learns the
MAC address of the sender from the source-hardware-address field in
the ARP request, and unicasts an ARP reply to the sender. The ARP
reply carries the replying node's MAC address in the source-hardware-
address field, thus enabling two-way communication between the two
nodes.
A variation of this scheme, known as "proxy ARP" [RFC2002], allows a
node to respond to an ARP request with its own MAC address, even when
the responding node does not own the requested IP address.
Generally, the proxy ARP response is generated by routers to attract
traffic for prefixes they can forward packets to. This scheme
requires the host to send ARP queries for the IP address the host is
trying to reach, rather than the IP address of the router. When
there is more than one router connected to a network, proxy ARP
enables a host to automatically select an exit router without running
any routing protocol to determine IP reachability. Unlike regular
ARP, a proxy ARP request can elicit multiple responses, e.g., when
more than one router has connectivity to the address being resolved.
The sender must be prepared to select one of the responding routers.
Yet another variation of the ARP protocol, called 'Gratuitous ARP'
[RFC2002], allows a node to update the ARP cache of other nodes in an
unsolicited fashion. Gratuitous ARP is sent as either an ARP request
or an ARP reply. In either case, the Source Protocol Address and
Target Protocol Address contain the sender's address, and the Source
Hardware Address is set to the sender's hardware address. In case of
a gratuitous ARP reply, the Target Hardware Address is also set to
the sender's address.
Kompella & Balaji Expires August 18, 2014 [Page 3]
Internet-Draft Label Distribution Using ARP February 2014
3. L-ARP Protocol Operation
The L-ARP protocol builds on the proxy ARP model, and also leverages
gratuitous ARP model for asynchronous updates.
In this memo, we will refer to L-ARP clients (that make L-ARP
requests) and L-ARP servers (that send L-ARP responses). In
Figure 1, C1, C2 and C3 are L-ARP clients, and S1, S2 and S3 are
L-ARP servers. T is a member of the MPLS Fabric that may not be an
L-ARP server. Within the MPLS Fabric, the usual MPLS protocols (IGP,
LDP, RSVP-TE) are run. Say C1, C2 and C3 want to establish MPLS
tunnels to each other (for example, they are using BGP MPLS VPNs as
the overlay virtual network technology). C1 might also want to talk
to a member of the MPLS Fabric, say T.
. . . . . .
. .
C1 --- S1 T
\ . MPLS .
\ . .
\ . Fabric .
C2 --- S2 S3 --- C3
. .
. . . . . .
Figure 1
3.1. Basic Operation
A node (say C1) that needs an MPLS tunnel to a destination (say C3)
broadcasts an L-ARP query with the Target Protocol Address set to C3.
A node that has reachability to C3 (such as S1 or S2) sends an L-ARP
reply with the Source Hardware Address set to a locally-allocated
MPLS label plus its Ethernet MAC address. After receiving one or
more L-ARP replies, C1 can select either S1 or S2 to send MPLS
packets that are destined to C3. As described later, the L-ARP
response may contain certain parameters that enable the client to
make an informed choice of the routers.
As with standard ARP, the validity of the MPLS label obtained using
L-ARP is time-bound. The client should periodically resend its L-ARP
requests to obtain the latest information, and time out entries in
its ARP cache if such an update is not forthcoming. Once an L-ARP
server has advertised a label binding, it MUST NOT change the binding
until expiry of the binding's validity time.
The mechanism defined here is simplistic; see Section 4.
Kompella & Balaji Expires August 18, 2014 [Page 4]
Internet-Draft Label Distribution Using ARP February 2014
3.2. Asynchronous operation
The preceding sections described a request-response based model. In
some cases, the L-ARP server may want to asynchronously update its
clients. L-ARP uses the gratuitous ARP model [RFC2002] to "push"
such changes.
In a pure "push" model, a device may send out updates for all
prefixes it knows about. This naive approach will not scale well.
This memo specifies a mode of operation that is somewhere between
"push" and "pull" model. An L-ARP server does not advertise any
binding for a prefix until at least one L-ARP client expresses
interest in that prefix (by initiating an L-ARP query). As long as
the server has at least one interested client for a prefix, the
server sends unsolicited (aka gratuitous, though the term is less
appropriate in this context) L-ARP replies when a prefix's
reachability changes. The server will deem the client's interest in
a prefix to have ceased when it does not hear any L-ARP queries for
some configured timeout period.
3.3. Applicability
L-ARP can be used between a server and its Top-of-Rack switch in a
Data Center. L-ARP can also be used between a DSLAM and its
aggregation switch going to the B-RAS. More generally, L-ARP can be
used between an "access node" and its first hop MPLS-enabled device
in the context of Seamless MPLS [reference]. In all these cases,
L-ARP can handle the presence of multiple connections between the
access device and its first hop devices.
ARP is not a routing protocol. The use of L-ARP should be limited to
cases where the L-ARP client has a small number of one-hop
connections to L-ARP servers. The presence of a complex topology
between the L-ARP client and server suggests to use a different
protocol.
4. For Future Study
The L-ARP specification is quite simple, and the goal is to keep it
that way. However, inevitably, there will be questions and features
that will be requested. Some of these are:
1. Keeping L-ARP clients and servers in sync. In particular,
dealing with:
A. client and/or server restart
B. lost packets
Kompella & Balaji Expires August 18, 2014 [Page 5]
Internet-Draft Label Distribution Using ARP February 2014
C. timeouts
2. Withdrawing a response.
3. Dealing with scale.
4. If there are many servers, which one to pick?
5. How can a client make best use of underlying ECMP paths?
6. and probably many more.
In all of these, it is important to realize that, whenever possible,
a solution that places most of the burden on the server rather than
on the client is preferable.
5. L-ARP Message Format
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$hrd | ar$pro |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ar$hln | ar$pln | ar$op |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$sha (variable...) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$spa (variable...) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$tha (variable...) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ar$tpa (variable...) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: L-ARP Packet Format
ar$hrd Hardware Type: MPLS-over-Ethernet. The value of the field
used here is [HTYPE-MPLS-TBD]. To start with, we will use the
experimental value HW_EXP2 (256)
ar$pro Protocol Type: IP. The value of the field used here is
0x0800.
ar$hln Hardware Length: the value of the field used here is 12.
ar$pln Protocol Address Length: the value is 4.
Kompella & Balaji Expires August 18, 2014 [Page 6]
Internet-Draft Label Distribution Using ARP February 2014
ar$op Operation Code: set to 1 for request, and 2 for reply.
ar$sha Source Hardware Address: In an L-ARP query message, Source
Hardware Address is irrelevant, and set to all-zeroes. In an
L-ARP reply message, the address follows the 'hardware address'
format specified below.
ar$spa Source Protocol Address: In an L-ARP query message, this
field carries the sender's IP address. In an L-ARP reply
message, this field carries the target protocol address received
in the corresponding query message.
ar$tha Target Hardware Address: This field is invalid in both
request and reply messages.
ar$tpa Target Protocol Address: In an L-ARP query message, this
field carries the IP address for which the client is seeking an
MPLS label. In an L-ARP reply message, this field carries the
Source Protocol Address received in the corresponding L-ARP
query.
The following diagram describes the format of 'Hardware Address'
carried in L-ARP.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (6 octets) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... MAC Address (contd) | MPLS Label ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |Z|Z|Z|Z|
+-+-+-+-+-+-+-+-+
Figure 3: MPLS Hardware Address Format
MAC Address This field contains the Ethernet hardware address that
data packets should be directed to.
MPLS Label This field contains the MPLS label allocated by the
server. This field is valid only in an L-ARP request message.
This field is 20 bits wide, left-justified.
Z These bits are not used, and SHOULD be set to zero on sending and
ignored on receipt.
Kompella & Balaji Expires August 18, 2014 [Page 7]
Internet-Draft Label Distribution Using ARP February 2014
If other parameters are deemed useful in the L-ARP reply, they will
be added as needed.
6. Security Considerations
TODO
7. IANA Considerations
TODO
8. Acknowledgments
Many thanks to Shane Amante for his detailed comments and
suggestions. Many thanks to the team in Juniper prototyping this
work for their suggestions on making this variant workable in the
context of existing ARP implementations.
9. Normative References
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC2002] Perkins, C., "IP Mobility Support", RFC 2002, October
1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
Authors' Addresses
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
USA
Email: kireeti@juniper.net
Kompella & Balaji Expires August 18, 2014 [Page 8]
Internet-Draft Label Distribution Using ARP February 2014
Balaji Rajagopalan
Juniper Networks, Inc.
Prestige Electra, Exora Business Park
Marathahalli - Sarjapur Outer Ring Road
Bangalore 560103
India
Email: balajir@juniper.net
Kompella & Balaji Expires August 18, 2014 [Page 9]
