Network Working Group George Swallow
Internet Draft Cisco Systems, Inc.
Category: Standards Track
Expiration Date: April 2004
Kireeti Kompella
Juniper Networks, Inc.
Dan Tappan
Cisco Systems, Inc.
October 2003
Label Switching Router Self-Test
draft-ietf-mpls-lsr-self-test-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a means of self test for a Label-Switching
Router (LSR) to verify that its dataplane is functioning for
certain key Multi-Protocol Label Switching (MPLS) applications
including unicast forwarding based on LDP [LDP] and traffic
engineering tunnels based on [RSVP-TE]. A new Loopback FEC type
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is defined to allow an upstream neighbor to assist in the testing
at very low cost. MPLS Echo Request and MPLS Echo Reply messages
[LSP-Ping] messages are extended to do the actually probing.
Contents
1 Introduction ........................................... 4
1.1 Conventions ............................................ 4
2 Loopback FEC ........................................... 5
2.1 Loopback FEC Element ................................... 5
2.2 LDP Procedures ......................................... 6
3 Data Plane Self Test ................................... 6
3.1 Next Hop Verification Object ........................... 7
3.2 Additional Error Codes ................................. 9
3.3 Sending procedures ..................................... 9
3.4 Receiving procedures ................................... 10
3.5 Upstream Neighbor Verification ......................... 11
4 Security Considerations ................................ 11
5 IANA Considerations .................................... 12
6 Acknowledgments ........................................ 12
7 References ............................................. 12
7.1 Normative References ................................... 12
7.2 Informative References ................................. 12
8 Authors' Addresses ..................................... 13
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0. Sub-IP ID Summary
(This section to be removed before publication.)
(See Abstract above.)
RELATED DOCUMENTS
May be found in the "references" section.
WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
Fits in the MPLS box.
WHY IS IT TARGETED AT THIS WG
MPLS WG is currently looking at MPLS-specific error detection and
recovery mechanisms. The mechanisms proposed here are for packet-
based MPLS LSPs, which is why the MPLS WG is targeted.
JUSTIFICATION
The WG should consider this document, as it allows network
operators to detect MPLS LSP data plane failures in the network.
This type of failures have occurred, and are a source of concern
to operators implementing MPLS networks.
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1. Introduction
This document defines a means of self test for a Label-Switching
Router (LSR) to verify that its dataplane is functioning for certain
key Multi-Protocol Label Switching (MPLS) applications including
unicast forwarding based on LDP [LDP] and traffic engineering tunnels
based on [RSVP-TE]. MPLS Echo Request and MPLS Echo Reply messages
[LSP-Ping] messages are extended to do the actually probing. The
pings are sent to an upstream neighbor, looped back through the LSR
under test and intercepted, by means of TTL expiration by a
downstream neighbor. Extensions to LSP-Ping are defined to allow the
down stream neighbor to verify the test results.
In order to minimize the load on upstream LSRs a new loopback FEC is
defined. 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.
Note that use of a loopback allows an LSR to label entries for which
the LSR is not currently its potential upstream neighbor's next hop.
In this way label entries can be verified prior to the occurrence of
a routing change.
Some routing protocls, most notably OSPF have no means of exchanging
"Link Local Identifiers" used to identify unnumbered links and
components of bundled links. These same 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 MPLS
Ping expire one hop sooner, i.e. at the testing LSR itself.
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].
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2. Loopback FEC
The Loopback FEC type is defined to enable an upstream neighbor to
assist in the LSR self-testing at very low cost. This FEC allows the
loopback to occur in the dataplane without control plane involvement
beyond the initial LDP exchange and dataplane setup.
An LSR uses the Loopback FEC to selectively advertise loopback labels
to its neighbor LSRs. Each loopback label is bound to a particular
interface. For multiaccess links, one label per 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 advertised, the loopback
label is popped and the packet is sent on the interface to which the
loopback label was bound.
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
follows: (note: 130 is provisionally assigned, the actual value will
be assigned by IANA.)
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 | Interface Type| Id Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Identifier |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
Must be set to zero on transmission and ignored on receipt.
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Interface Type
# Type Interface Identifier
--- ---- --------------------
0 Unnumbered A 32 bit Link Identifier as
defined in [RFC3477]
1 IPv4 Numbered IPv4 Address
2 IPv6 Numbered IPv6 Address
Identifier Length
Length of the interface identifier in octets.
Address
An identifier encoded according to the Identifier Type field.
The length is 4 bytes for Unnumbered and IPv4, 16 bytes for
IPv6.
2.2. LDP Procedures
It is RECOMMENDED that loopback labels only be distributed in
response to a Label Request message, irrespective of the label
advertisement 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 neighbors link
identifier to form the request. In these cases, the downstream LSR
will need to 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 neighbor. We refer to these as
LSRs T, U, and D respectively. The packet flow is shown below.
Although the figure shows LSRD adjacent to LSRT it may in some cases
be an arbitrary number of hops away.
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+------+ +------+ +------+
| ,-|-------|-<MERq| | | MERq: MPLS Echo Request
| `-|-------|------|-------|-> |
| | | <-|-------|-<MERp| MERp: MPLS Echo Reply
+------+ +------+ +------+
LSRU LSRT LSRD
Figure 1: Self Test Message Flow
In order to perform a test on an incoming label stack, LSRT forms an
MPLS Echo Request. Included in that is a Next Hop Verification
Object which describes the interface and label stack that should be
seen by LSRD. Optionally a FEC Stack TLV may be included to verify
LSRD's labels are mapped to the expected FECs.
LSRT prepends the packet with the incoming label stack and the
loopback label received from LSRU. The TTL values are set such that
they will expire at LSRD. LSRT then forward the packet to LSRU.
LSRU receives the packet and performs normal MPLS forwarding, that is
the loopback label is pop, the TTL is decremented and propagated (in
this case) to the exposed label.
LSRT receives the packet and performs normal MPLS forwarding. If
everything is functioning as expected this will cause the packet to
arrive at LSRD with a TTL of 1.
LSRD receives the packet. It verifies that the packet was received
on the interface and with the label stack contained in the Next Hop
Verification TLV. If a FEC Stack TLV is also included, it verifies
that the labels in the stack are mapped to those FECs. The results
are recorded in an MPLS Echo Reply message and sent to LSRT.
3.1. Next Hop Verification Object
The Next Hop Verification is an optional TLV in an echo request. The
Length is 12 + 4*N octets, where N is the number of Downstream
Labels. The value field of a Next Hop Verification TLV 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop IPv4 Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Address Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Label | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Label | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Next Hop Interface Address Type is one of:
Type # Address Type
------ ------------
1 IPv4
2 Unnumbered
Reserved
Must be set to zero on transmission and ignored on receipt.
[Note a format for IPv6 will be added after one is defined in
[LSP-Ping]]
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3.2. Additional Error Codes
The following error codes are defined in addition to those defined in
[LSP-Ping].
Value Meaning
----- -------
8 Incorrect interface
9 Label Stack mis-match
10 Label not valid on this interface
11 Binding for this label is not the given FEC
12 Not processed
3.3. Sending procedures
In order to perform a test on an incoming label stack, an LSR first
determines the expected outgoing label stack, next hop router and
next hop interface. These are recorded in a Next Hop Verification
TLV.
If binding verification is also to be performed, then beginning at
the top of the stack, for each label binding received or given to the
Next Hop router, a corresponding entry in a FEC Stack TLV is
included. If there are labels in the stack that are not to be
processed by the Next Hop Router, corresponding entries MUST NOT be
included in the FEC Stack TLV.
The LSR creates an MPLS Echo Request packet with itself as the source
address and the destination set to an address in the range of 127/8.
The IP TTL SHOULD be set to 1. The incoming label stack is prepended
to the packet. The TTL of these labels SHOULD be set to appropriate
values - 2 for those labels which will be process by itself when the
packet is looped back; 1 for those labels which will be carried
through. Finally the loopback label received for the incoming
interface is prepended to the packet. 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.
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3.4. Receiving procedures
The next hop router performs the following checks on the received
packet.
1. Verify that the Next Hop IPv4(v6) Router ID matches one of its IP
addresses.
If this fails, return an error code 5, 'Replying router is not
one of the Downstream Routers'
2. Verify that the Next Hop Interface Address matches the interface
on which this packet arrived.
If this fails, return an error code xx1, 'Incorrect interface'.
A Next Hop Verification TLV SHOULD be included in the reply with
the received interface and label stack.
3. Verify that the label stack on the packet matches the label stack
in the Next Hop Verification TLV.
If this fails, return an error code xx2, 'Label Stack mis-match'.
A Next Hop Verification TLV SHOULD be included in the reply with
the received interface and label stack.
4. Verify that the top label on the stack is valid on this
interface.
If this fails, return an error code xx3, 'Label not valid on this
interface'. The error sub-code is set to the stack depth of the
errant label.
4a. If the label operation in step 4 is a Pop and requires that the
next label be inspected, repeat step 4 for that label.
5. If a FEC Stack TLV is present, then for each FEC verify that the
corresponding label is the correct binding. If the label binding
is incorrect, return an error code of xx4 'Binding for this label
is not the given FEC'. The error sub-code is set to the stack
depth of the errant label.
If a FEC is reached that was not processed in step 4 above,
return an error code of xx5, 'Not processed'. The error sub-code
is set to the stack depth of the errant label.
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3.5. Upstream Neighbor Verification
To verify that an upstream neighbor is properly echoing packets an
LSR may send an MPLS Echo 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 processing the loopback
label, it also allow the node to verify the neighbor's interface
mapping.
+------+ +------+ +------+
| ,-|-------|-<MERq| | | MERq: MPLS Echo Request
| `-|-------|-> | | |
| | | | | |
+------+ +------+ +------+
LSRU LSRT LSRD
Figure 2: Upstream Neighbor Verification
No TLVs need to be included in the MPLS Echo Request. By noting the
Sender's Handle and Sequence Number, as well as the loopback label,
LSRT is able to detect that a) the packet was looped, and b)
determine (or verify) the interface on which the packet was received.
A Next Hop Verification TLV may be included to assist in
verification. This may be particularly useful in a system where
control is distributed over multiple processor.
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 the Global Label Space, or any other label space shared across
multiple LDP sessions, it is RECOMMENDED that all loopback label 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
TBD
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
[RFC3036] Andersson, L. et al., "LDP Specification", January 2001.
[LSP-Ping] Bonica, R. et al., "Detecting MPLS Data Plane Liveness",
work-in-progress.
[RFC3477] Kompella, K. & 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.
7.2. Informative References
[RSVP-TE] Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC 3209, December 2001.
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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
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
Email: tappan@cisco.com
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