Network Working Group George Swallow
Internet Draft Cisco Systems, Inc.
Category: Standards Track
Expiration Date: November 2007
Kireeti Kompella
Juniper Networks, Inc.
Dan Tappan
May 2007
Label Switching Router Self-Test
draft-ietf-mpls-lsr-self-test-07.txt
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Abstract
This document defines a means for a Label-Switching Router to
verify that its data plane is functioning for certain key Multi-
Protocol Label Switching applications, including unicast
forwarding and traffic engineering tunnels. A new Loopback FEC
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type is defined to allow an upstream neighbor to assist in the
testing at very low cost. MPLS Verification Request and MPLS
Verification Reply messages are defined to do the actual probing.
Contents
1 Introduction .............................................. 3
1.1 Conventions ............................................... 4
2 Loopback FEC .............................................. 4
2.1 Loopback FEC Element ...................................... 4
2.2 LDP Procedures ............................................ 6
3 Data Plane Self Test ...................................... 6
3.1 Data Plane Verification Request / Reply Messages .......... 7
3.2 UDP Port .................................................. 9
3.3 Reply-To Address Object ................................... 9
3.3.1 IPv4 Reply-To Address Object .............................. 9
3.3.2 IPv6 Reply-To Address Object .............................. 9
3.4 Sending procedures ........................................ 10
3.5 Receiving procedures ...................................... 11
3.6 Upstream Neighbor Verification ............................ 12
4 Security Considerations ................................... 12
5 IANA Considerations ....................................... 13
6 Acknowledgments ........................................... 13
7 References ................................................ 13
7.1 Normative References ...................................... 13
7.2 Informative References .................................... 14
8 Authors' Addresses ........................................ 14
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1. Introduction
This document defines extentions to RFC 4379 [LSP-PING] (which is
generally known as Label-Switched-Path (LSP) Ping) to provide a means
for a Label-Switching Router (LSR) to verify that its data plane is
functioning for certain key Multi-Protocol Label Switching (MPLS)
applications, including unicast forwarding based on the Label Distri-
bution Protocol (LDP) [LDP] and traffic engineering tunnels based on
[RSVP-TE]. MPLS Verification Request and MPLS Verification Reply
messages are defined to do the actual probing. The pings are sent to
an upstream neighbor, looped back through the LSR under test and
intercepted, by means of time-to-live (TTL) expiration by a down-
stream neighbor.
In order to minimize the load on upstream LSRs a loopback FEC Type is
defined. Labels advertised with this FEC Type are referred to as
loopback labels. Receipt of a packet labeled with a loopback label
will cause the advertising LSR to pop the label off the label stack
and send the packet out the advertised interface.
Use of a loopback mechnism allows an LSR to test label entries which
are not currently in use. For example many LSRs advertise label map-
pings for all IPv4 routes to all of their neighbors. For some por-
tion of these their neighbor LSR is not currently upstream and the
label entry is not used. But if the neighbors best path to a desti-
nation changes, that route and the associated label entry will be
used. An LSR can loop traffic through a "non-upstream" LSR because
that LSR is acting only on the loopback label and not on the underly-
ing label associated with the actual forwarding equivalence class
(FEC) being tested. In this way label entries can be verified prior
to the occurrence of a routing change.
Some routing protocols, most notably Open Shortest-Path-First (OSPF)
[OSPF] have no means of exchanging the "Link Local Identifiers" used
to identify unnumbered links and components of bundled links. These
test procedures can be used to associate the neighbor's interfaces
with the probing LSRs interfaces. This is achieved by simply having
the TTL of the LSP Ping expire one hop sooner, i.e. at the testing
LSR itself.
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1.1. Conventions
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 [KEYWORDS].
2. Loopback FEC
The Loopback FEC type is defined to enable an upstream neighbor to
assist in LSR self-testing at very low cost. This FEC causes the
loopback to occur in the data plane without control plane involvement
beyond the initial LDP exchange and data-plane setup. The FEC also
carries information to indicate the desired encapsulation should it
be the only label in a received label stack. Values are defined for
IPv4 and IPv6.
An LSR uses a Loopback FEC to selectively advertise loopback labels
to its neighbor LSRs. Each loopback label is bound to a particular
interface. For multi-access links, a unique label for each neighbor
is required, since the link-level address is derived from the label
lookup. When an MPLS packet with its top label set to a loopback
label is received from an interface over which that label was adver-
tised, the loopback label is popped and the packet is sent on the
interface to which the loopback label was bound. If the label-stack
only contains the one loopback label, the encapsulation of the packet
is determined by the FEC Type.
TTL treatment for loopback labels follows the Uniform model. I.e.
the TTL carried in the loopback label is decremented and copied to
the exposed label or IP header as the case may be.
2.1. Loopback FEC Element
FEC element type 130 is used. The FEC element is encoded as fol-
lows: (note: 130 is provisionally assigned, the actual value will be
assigned by IANA.)
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0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 130 | Res | If & Prot Type| Id Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Identifier |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved (Res)
MUST be set to zero on transmission and ignored on receipt.
Interface & Protocol Type
# Type Interface Identifier
--- ---- --------------------
1 IPv4 Numbered IPv4 Address
2 IPv4 Unnumbered A 32 bit Link Identifier as
defined in [RFC3477]
3 IPv6 Numbered IPv6 Address
4 IPv6 Unnumbered A 32 bit Link Identifier as
defined in [RFC3477]
Note that these type values also indicate the encapsulation
(IPv4 or IPv6) for payloads that have a label stack containing
only a loopback label.
Identifier Length
Length of the interface identifier in octets. The length is 4
bytes for the unnumbered types and IPv4, 16 bytes for IPv6.
Address
An identifier encoded according to the Identifier Type field.
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2.2. LDP Procedures
It is RECOMMENDED that loopback labels only be distributed in
response to a Label Request message, irrespective of the label adver-
tisement mode of the LDP session. However it is recognized that in
certain cases such as OSPF with unnumbered links, the upstream LSR
may not have sufficiently detailed information of the neighbor's link
identifier to form the request. In these cases, the downstream LSR
MAY be configured to make unsolicited advertisements.
3. Data Plane Self Test
A self test operation involves three LSRs, the LSR doing the test, an
upstream neighbor and a downstream LSR. Upstream here is with
respect to the flow of the test (which in some cases could be differ-
ent than the normal sense of upstream in IP routing). We refer to
these as LSRs T, U, and D respectively. In order to minimize the
processing load on LSR-D, two new LSP Ping messages are defined,
called the MPLS Data Plane Verification Request and the MPLS Data
Plane Verification Reply. These messages are used to allow LSR-T to
obtain the label stack, address and interface information of LSR-D.
The packet flow is shown below. Although the figure shows LSR-D adja-
cent to LSR-T it may in some cases be an arbitrary number of hops
away.
+-------+ +-------+ +-------+
| ,-|-------|<DPVRq | | |
| `-|-------|-------|-------|-> |
| | | | | |
| | | <-|-------|<DPVRp |
+-------+ +-------+ +-------+
LSR-U LSR-T LSR-D
DPVRq: MPLS Data Plane Verification Request
DPVRp: MPLS Data Plane Verification Reply
Figure 1: Self Test Message Flow
In order to perform a test on an incoming label stack, LSR-T forms an
MPLS Data Plane Verification Request. LSR-T prepends the packet with
the incoming label stack being tested and the loopback label received
from LSR-U. The TTL values are set such that they will expire at
LSR-D. LSR-T then forwards the packet to LSR-U.
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LSR-U receives the packet and performs normal MPLS forwarding. That
is, the loopback label is popped, the TTL is decremented and propa-
gated (in this case) to the exposed label.
LSR-T receives the packet and performs normal MPLS forwarding. If
everything is functioning as expected this will cause the packet to
arrive at LSR-D with a TTL of 1.
In this example, we assume that all is working properly. The TTL
expires at LSR-D causing it to receive the packet. LSR-D notes the
the interface and the label stack on which the packet was received
and records these in an Interface and Label Stack TLV. This Object
is sent to LSR-T in an MPLS Data Plane Verification Reply message.
3.1. Data Plane Verification Request / Reply Messages
Two new LSP Ping messages are defined for LSR self test. The purpose
of the new messages is three fold. First the timestamps are removed
to minimize processing. Second the message type allows simple recog-
nition that minimal processing is necessary to service this request.
Third, the Verification Request message itself conveys the the
request, thus a Verification Request message with no Objects is both
legal and normal.
The definitions of all fields in the messages are identical to those
found in [LSP-PING].
The new message types are: (Provisionally; to be assigned)
Type Message
---- -------
3 MPLS Data Plane Verification Request
4 MPLS Data Plane Verification Reply
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The messages have the following format:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Number | MUST Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reply mode | Return Code | Return Subcode|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs ... |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MPLS Data Plane Verification Request message MAY contain the fol-
lowing objects:
Type # Object
------ -----------
3 Pad
10 Reply TOS Byte
11 (provisional) IPv4 Reply-to Address
12 (provisional) IPv6 Reply-to Address
64512-65535 Vendor Private TLVs
The MPLS Data Plane Verification Reply message MAY contain the fol-
lowing objects:
Type # Object
------ -----------
7 Interface and Label Stack
9 Errored TLVs
64512-65535 Vendor Private TLVs
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3.2. UDP Port
MPLS Data Plane Verification Request messages MAY be sent to port
3503 as is used for [LSP-PING]. However to aid implementations that
wish to handle these messages at a lower level than MPLS Echo Request
messages another UDP port, <tbd>, is provided. Port <tbd> SHOULD be
used by default. The source UDP port, as in [LSP-PING] is chosen by
the sender.
3.3. Reply-To Address Object
In order to perform detailed diagnostics of a particular failing flow
in the face of ECMP, it is useful to be able to use the exact source
and destination addresses of that flow. The Reply-To Object is an
optional TLV in a MPLS Data Plane Verification Request message. The
Object has two formats, type 11 for IPv4 and type 12 for IPv6 (to be
assigned by IANA).
3.3.1. IPv4 Reply-To Address Object
The length of an IPv4 Reply-to Address object is 4 octets; the value
field has the following format:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-to IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reply-to IPv4 Address
The address to which the MPLS Data Plane Verification Reply
message is to be sent.
3.3.2. IPv6 Reply-To Address Object
The length of an IPv6 Reply-to Address object is 16 octets; the value
field has the following format:
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0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-to IPv6 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-to IPv6 Address (Cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-to IPv6 Address (Cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-to IPv6 Address (Cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reply-to IPv6 Address
The address to which the MPLS Data Plane Verification Reply
message is to be sent.
3.4. Sending procedures
In order to perform a test on an incoming labeled or unlabeled
packet, an LSR first determines the expected outgoing label stack,
next hop router and next hop interface.
The LSR creates an MPLS Data Plane Verification Request message.
In normal use, the source address is set to an address belonging to
the LSR and the destination set to an address in the range of 127/8.
The incoming label stack (if any) is prepended to the packet. The
TTL of these labels and the packet header SHOULD be set to appropri-
ate values - 2 for those labels and/or header which will be processed
by this node when the packet is looped back; 1 for those labels
and/or header which will be carried through. Finally the loopback
label bound to the incoming interface is prepended to the packet. In
the case of an otherwise unlabeled packet the label's FEC MUST indi-
cate the appropriate IP version. The TTL is set such that it will
have the value of 3 on the wire.
The packet is sent to the upstream neighbor on an interface for which
the loopback label is valid.
In diagnostic situations, the source and destination addresses MAY be
set to any value. In this case, a Reply-to IPv4 or IPv6 Address
object MUST be included. The IP TTL MUST be set to 1. The TTL of
labels other than the loopback label MUST be set to appropriate val-
ues - 2 for those labels which will be process by this LSR when the
packet is looped back; 1 for those labels which will be carried
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through.
In some MPLS deployments TTL hiding is used to make a providers net-
work appear as a single hop. That is the TTL in the imposed label
does not reflect the TTL of the received packet. It is RECOMMENDED
that testing of label imposition SHOULD NOT be performed in such cir-
cumstances as the Verification Request will in most case travel mul-
tiple hops.
3.5. Receiving procedures
An LSR X that receives an MPLS Verification Request message formats a
MPLS Verification Reply message. The Sender's Handle and Sequence
Number are copied from the Request message.
X then parses the packet to ensure that it is a well-formed packet,
and that the TLVs that are not marked "Ignore" are understood. If
not, X SHOULD set the Return Code set to "Malformed echo request
received" or "TLV not understood" (as appropriate), and the Subcode
set to zero. In the latter case, the misunderstood TLVs (only) are
included in the reply.
If the Verification Request is good, X MUST note the interface and
label stack of the received Verification Request and format this
information as a Downstream Verification object. This object is
included in the MPLS Verification Reply message. The Return Code and
Subcode MUST be set to zero, indicating "No return code".
The source address of the Reply message MUST be an address of the
replying LSR. If the request included a Reply-to IPv4 or IPv6
Address object, the MPLS Data Plane Verification Reply message MUST
be sent to that address. Otherwise the Reply message is sent to the
source address of the Verification Request message.
An LSR MUST be capable of filtering addresses that are to be replied
to. If a filter has been invoked (i.e. configured) and an address
does not pass the filter, then a reply MUST NOT be sent, and the
event SHOULD be logged.
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3.6. Upstream Neighbor Verification
To verify that an upstream neighbor is properly echoing packets an
LSR may send an MPLS Data Plane Verification Request packet with the
TTL set so that the packet will expire upon reaching reaching itself.
This procedure not only tests that the neighbor is correctly process-
ing the loopback label, it also allows the node to verify the neigh-
bor's interface mapping.
+-------+ +-------+
| | | |
| ,-|-------|<DPVRq |
| `-|-------|-> |
| | | |
+-------+ +-------+
LSR-U LSR-T
DPVRq: MPLS Data Plane Verification Request
Figure 2: Upstream Neighbor Verification
No TLVs need to be included in the MPLS Data Plane Verification
Request. By noting the Sender's Handle and Sequence Number, as well
as the loopback label, LSR-T is able to detect that a) the packet was
looped, and b) determine (or verify) the interface on which the
packet was received.
4. Security Considerations
Were loopback labels widely known, they might be subject to abuse.
It is therefore RECOMMENDED that loopback labels only be shared
between trusted neighbors. Further, if the loopback labels are drawn
from a per-platform label space, or any other label space shared
across multiple LDP sessions, it is RECOMMENDED that all loopback
labels be filtered from a session except those labels pertaining to
interfaces directly connected to the neighbor participating in that
session.
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5. IANA Considerations
This document makes the following codepoint assigments (pending IANA
action):
Registry Codepoint Purpose
UDP Port tbd MPLS Verification Request
LSP Ping Message Type 3 MPLS Data Plane Verification
Request
LSP Ping Message Type 4 MPLS Data Plane Verification
Reply
LSP Ping Object Type 11 IPv4 Reply-to Address
LSP Ping Object Type 12 IPv6 Reply-to Address
6. Acknowledgments
The authors would like to thank Vanson Lim, Tom Nadeau, and Bob
Thomas for their comments and suggestions.
7. References
7.1. Normative References
[LDP] Andersson, L. et al., "LDP Specification", RFC 3036,
January 2001.
[LSP-PING] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
in Resource ReSerVation Protocol - Traffic Engineering
(RSVP-TE)", January 2003.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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7.2. Informative References
[RSVP-TE] Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC 3209, December 2001.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
8. Authors' Addresses
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: kireeti@juniper.net
George Swallow
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
Email: swallow@cisco.com
Dan Tappan
Boxborough, MA 01719
Email: dtappan@alum.mit.edu
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