Internet DRAFT - draft-ietf-mpls-lsp-ping-reply-mode-simple
draft-ietf-mpls-lsp-ping-reply-mode-simple
Internet Engineering Task Force N. Akiya
Internet-Draft Big Switch Networks
Updates: 7110 (if approved) G. Swallow
Intended status: Standards Track C. Pignataro
Expires: April 10, 2016 Cisco Systems
L. Andersson
M. Chen
Huawei
October 8, 2015
Label Switched Path (LSP) Ping/Traceroute Reply Mode Simplification
draft-ietf-mpls-lsp-ping-reply-mode-simple-05
Abstract
The Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
Ping and Traceroute use the Reply Mode field to signal the method to
be used in the MPLS echo reply. This document updates the procedures
for the "Reply via Specified Path" Reply Mode, the value of this
Reply Mode is 5. The update creates a simple way to indicate that
the Reverse LSP should be used as return path. This document also
adds an optional TLV which can carry ordered list of Reply Mode
values.
This document updates RFC7110.
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 RFC 2119 [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."
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This Internet-Draft will expire on April 10, 2016.
Copyright Notice
Copyright (c) 2015 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statements . . . . . . . . . . . . . . . . . . . . . 3
3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Reply via Specified Path Update . . . . . . . . . . . . . 5
3.2. Reply Mode Order TLV . . . . . . . . . . . . . . . . . . 6
4. Relations to Other LSP Ping/Trace Features . . . . . . . . . 8
4.1. Backwards Compatibility with Reply via Specified Path
Reply Mode . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Reply Path TLV . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Example 1: Reply Mode Order TLV Usage with Reply Path
TLV . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Example 2: Reply Mode Order TLV Usage with Reply Path
TLV . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Proxy LSP Ping . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Proxy LSR Sending an MPLS Echo Request . . . . . . . 10
4.3.2. Proxy LSR Sending an MPLS Proxy Ping Reply . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Manageability Considerations . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. New Reply Mode Order TLV . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
9. Contributing Authors . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Reply Mode Order TLV Beneficial Scenarios . . . . . 13
A.1. Incorrect Forwarding Scenario . . . . . . . . . . . . . . 14
A.2. Non-Co-Routed Bidirectional LSP Scenario . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
The Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
Ping, described in [RFC4379], allows an initiator LSR to encode
instructions (Reply Mode) on how a responder LSR should send the
response back to the initiator LSR. [RFC7110] also allows the
initiator LSR to encode a TLV (Reply Path TLV) which can instruct the
responder LSR to use a specific LSP to send the response back to the
initiator LSR. Both approaches are powerful as they provide the
ability for the initiator LSR to control the return path.
However, it is becoming increasingly difficult for an initiator LSR
to select a valid return path to encode in the MPLS LSP echo request
packets. If the initiator LSR does not select a valid return path,
the MPLS LSP echo reply will not get back to the initiator LSR. This
results in a false failure of MPLS LSP Ping and Traceroute operation.
In an effort to minimize such false failures, different
implementations have chosen different default return path encoding
for different LSP types and LSP operations. The problem with
implementations having different default return path encoding is that
the MPLS echo reply will not work in many cases, and the default
value may not be the preferred choice by the operators.
This document describes:
o In Section 2, further description of the problems;
o In Section 3, a solution to minimize false failures while
accommodating operator preferences;
o In Section 4, relationships to other LSP Ping/Traceroute features;
o In Appendix A, examples of scenarios where the mechanism described
in this document provides benefits.
This document updates [RFC7110] by allowing the usage of the "Reply
via Specified Path" (value=5) Reply Mode without including the Reply
Path TLV. The update creates a simple way to indicate that the
Reverse LSP should be used as return path.
2. Problem Statements
It is becoming increasingly difficult for implementations to
automatically supply a workable return path encoding for all MPLS LSP
Ping and Traceroute operations across all LSP types. There are
several factors which are contributing to this complication.
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o Some LSPs have a control-channel, and some do not. Some LSPs have
a reverse LSP, and some do not. Some LSPs have IP reachability in
the reverse direction, and some do not.
o LSRs on some LSPs can have different available return path(s).
Available return path(s) can depend on whether the responder LSR
is a transit LSR or an egress LSR. In case of a bi-directional
LSP, available return path(s) on transit LSRs can also depend on
whether the LSP is completely co-routed, partially co-routed or
associated (i.e., the LSPs in the two directions are not co-
routed).
o MPLS echo request packets may incorrectly terminate on an
unintended target, which can have different available return
path(s) than the intended target.
o The MPLS LSP Ping operation is expected to terminate on an egress
LSR. However, the MPLS LSP Ping operation with specific TTL
values and MPLS LSP Traceroute operation can terminate on both
transit LSR(s) and the egress LSR.
Except for the case where the responder LSR does not have an IP route
back to the initiator LSR, it is possible to use the "Reply via an
IPv4/IPv6 UDP packet" (value=2) Reply Mode value in all cases.
However, some operators are preferring control-channel and reverse
LSP as default return path if they are available, which is not always
the case.
When specific return path encoding is supplied by users or
applications, then there are no issues in choosing the return path
encoding. When specific return path encoding is not supplied by
users or applications, then implementations use extra logic to
compute, and sometimes guess, the default return path encodings. If
a responder LSR receives an MPLS echo request containing return path
instructions which cannot be accommodated due to unavailability, then
the responder LSR often drops such packets. This failure mode
results in the initiator LSR not receiving the intended MPLS LSP echo
reply packets. The scenario described here is a potentially
acceptable result in some failure cases, like a broken LSP, where the
MPLS echo request terminated on an unintended target. However, if
the initiator LSR does not receive an MPLS echo replay, even after
the responder LSR receives the MPLS echo request and is able to
verify the request, information is sent back to the user(s) which is
considered a false failure.
Many operators prefer particular return path(s) over others return
path(s) for specific LSP types. To accommodate operator preferred
paths, implementations may default to operator preferred return paths
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for particular operations, or allow a default return path to be
configured. It would not be considered beneficial to use a preferred
return path for an intended target LSR if there is previous knowledge
at the initiator LSR that the return path is not available. Using a
unavailable preferred return path would undesirably result in the
initiator LSR not receiving the MPLS echo return packets. It would
be considered beneficial, for given operations, if the sender of the
MPLS echo request would be able to determined return path
availability before the operation is initiated.
This document updates the procedures for "Reply via Specified Path"
Reply Mode to easily indicate the reverse LSP, and adds one optional
TLV to describe an ordered list of Reply Modes. Based on operational
needs, the TLV can describe multiple Reply Mode values in a preferred
order to allow the responder LSR to use the first available Reply
Mode from the list. This eliminates the need for the initiator LSR
to compute, or sometimes guess, the default return path encoding.
This new mode of operation would resulted in a simplification to
implementations across the various vendors and improve both usability
and operational needs.
3. Solution
This document updates the procedures for "Reply via Specified Path"
Reply Mode to easily indicate the reverse LSP. This document also
adds an optional TLV which can carry an ordered list of Reply Modes.
3.1. Reply via Specified Path Update
Some LSP types are capable of having a related LSP in reverse
direction, through signaling or other association mechanisms.
Examples of such LSP types are bidirectional Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) LSPs [RFC3473] and MPLS
Transport Profile (MPLS-TP) LSPs ([RFC5960]). This document uses the
term "Reverse LSP" to refer to the LSP in the reverse direction of
such LSP types. Note that this document restricts the scope of
"Reverse LSP" applicability to those reverse LSPs which are capable
and allowed to carry the IP encapsulated MPLS echo reply.
[RFC7110] has defined the Reply Mode "Reply via Specified Path" which
allows the initiator LSR to instruct the responder LSR to send the
MPLS echo reply message on the reverse LSP. However, the instruction
also requires the initiator LSR to include the "Reply Path TLV" with
the B bit (Bidirectional bit) set in the Flags field. Additionally,
[RFC7110] defines that if the "Reply via Specified Path" Reply Mode
is used the "Reply Path TLV" MUST present.
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This document updates the procedures for "Reply via Specified Path"
Reply Mode as follows:
o The "Reply via Specified Path" MAY be used without including a
"Reply Path TLV".
o The usage of the "Reply via Specified Path" without inclusion of a
"Reply Path TLV" implies the reverse LSP. In other words, the
usage of the "Reply via Specified Path" without inclusion of a
"Reply Path TLV" has the same semantics as the usage of the "Reply
via Specified Path" with inclusion of a "Reply Path TLV" with the
B bit set in the Flags field.
Specific to section 5.1 of [RFC7110], this document updates the first
sentence as follows:
o When sending an echo request, in addition to the rules and
procedures defined in Section 4.3 of [RFC4379], the Reply Mode of
the echo request MUST be set to "Reply via Specified Path", and a
Reply Path TLV SHOULD be carried in the echo request message
correspondingly; if the Reply Path TLV is not carried, then it
indicates the reverse LSP as the reply path.
Note that the reverse LSP is in relation to the last FEC specified in
the Target FEC Stack TLV.
3.2. Reply Mode Order TLV
This document also introduces a new optional TLV to describe a list
of Reply Mode values. The new TLV will contain one or more Reply
Mode value(s) in preferred order. The first Reply Mode value is the
most preferred and the last Reply Mode value is the least preferred.
Following rules apply when using Reply Mode Order TLV.
1. The Reply Mode Order TLV MUST NOT be included in any MPLS echo
reply. If the initiator LSR receives an MPLS echo reply with the
Reply Mode Order TLV, the initiator LSR MUST ignore the whole
Reply Mode Order TLV and MUST only use the value from the Reply
Mode field of the received MPLS echo reply. It may be beneficial
for implementations to provide counters and/or loggings, with
appropriate log dampening, to record this error case.
2. The Reply Mode Order TLV MAY be included in MPLS echo request.
3. The Reply Mode field of an MPLS echo request MUST be set to a
valid value even when supplying the Reply Mode Order TLV. The
initiator LSR SHOULD set the Reply Mode field of an MPLS echo
request to a value that corresponds to a return path which most
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likely to be available, in case the responder LSR does not
understand the Reply Mode Order TLV.
4. If a responder LSR understands the Reply Mode Order TLV but the
TLV is not valid (due to conditions described in the items 6, 7,
8 and 9 immediately below), then the responder LSR MUST ignore
the whole Reply Mode Order TLV and MUST only use the value from
the Reply Mode field of the received MPLS echo request. It may
be beneficial for implementations to provide counters and/or
loggings, with appropriate log dampening, to record this error
case.
5. If a responder LSR understands the Reply Mode Order TLV and the
TLV is valid, then the responder LSR MUST consider the Reply Mode
values described in the TLV and MUST NOT use the value described
in the Reply Mode field of the received MPLS echo request. In
other words, a valid Reply Mode Order TLV overrides the value
specified in the Reply Mode field of the received MPLS echo
request.
6. Reply Mode Order TLV MUST contain at least one Reply Mode value.
7. A Reply Mode value, except for Reply Mode value 5 (Reply via
Specified Path), MUST NOT be repeated (i.e., MUST NOT appear
multiple times) in the Reply Mode Order TLV.
8. The Reply Mode value 5 (Reply via Specified Path) MAY be included
more than once in the Reply Mode Order TLV. However, in such
case a Reply Path TLV MUST be included for all instances of the
Reply Mode value 5 included in the Reply Mode Order TLV. In
other words, 3 instances of the Reply Mode value 5 in the Reply
Mode Order TLV will require 3 instances of the Reply Path TLVs.
9. The Reply Mode value 1 (Do not reply) MUST NOT be used in the
Reply Mode Order TLV.
The responder LSR SHOULD select the first available return path in
this TLV. The Reply Mode value corresponding to the selected return
path MUST be set in Reply Mode field of the MPLS echo reply to
communicate back to the initiator LSR which return path was chosen.
The format of the TLV is as follows:
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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 Mode Order TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Reply Mode 1 | Reply Mode 2 | Reply Mode 3 | Reply Mode 4 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 Reply Mode Order TLV
This is a variable length optional TLV. The Reply Mode Order TLV
Type is TBD1.
The Length field is 2 octets in length. It defines the length in
octets of the list of Reply Mode values.
Each Reply Mode field is 1 octet, and there is no padding.
4. Relations to Other LSP Ping/Trace Features
4.1. Backwards Compatibility with Reply via Specified Path Reply Mode
[RFC7110] introduces the "Reply via Specified Path" (value=5) Reply
Mode. The RFC also defines that if this Reply Mode is used, the
"Reply Path TLV" MUST be included. This document relaxes the
semantics and defines that this Reply Mode MAY be used without the
"Reply Path TLV". This MAY be done to indicate that the reverse LSP
SHALL be used as he return path.
If the initiator LSR, which sent an MPLS echo request message with
the "Reply via Specified Path" Reply Mode but without including the
"Reply Path TLV", receives back an MPLS echo reply message with the
return code being "Malformed echo request received", then the
initiator LSR SHOULD assume that the responder LSR does not support
the mechanism defined in this document.
4.2. Reply Path TLV
A "Reply Path TLV" [RFC7110] is defined to identify a single return
path. When the initiator LSR wants to use the Reply Mode Order TLV
to describe multiple return paths, then the initiator SHOULD include
multiple "Reply via Specified Path" (value=5) Reply Mode values and
multiple corresponding "Reply Path TLV" objects (one "Reply Path TLV"
corresponding to a "Reply via Specified Path" Reply Mode, and one
"Reply Path TLV" identifies a return path).
As described in Section 3.1, it's valid to use the "Reply via
Specified Path" Reply Mode without inclusion a "Reply Path TLV". For
the Reply Mode Order TLV, it's also valid to include a "Reply via
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Specified Path" Reply Mode value without a corresponding "Reply Path
TLV", this implies that the reverse LSP is the preferred return path.
When multiple consecutive "Reply via Specified Path" Reply Mode
values are included but less corresponding "Reply Path TLV" objects
exist, the responder LSR SHOULD think that the former "Reply via
Specified Path" Reply Mode values have corresponding "Reply Path
TLV", the latter "Reply via Specified Path" Reply Mode values have no
corresponding "Reply Path TLV". For example, if the Reply Mode Order
TLV carrying Reply Modes {5, 5, 5} and only two Reply Path TLVs
carrying FEC X and FEC Y respectively. The reply path order is as
follows:
1. Reply via Specified Path (FEC X)
2. Reply via Specified Path (FEC Y)
3. Reply via Specified Path (Reverse LSP)
4.2.1. Example 1: Reply Mode Order TLV Usage with Reply Path TLV
If the initiator LSR was interested in encoding following return
paths:
1. Reply via application level control channel
2. FEC X
3. FEC Y
4. Reply via an IPv4/IPv6 UDP packet
Then the MPLS echo request message is to carry:
o The Reply Mode Order TLV carrying Reply Modes {4, 5, 5, 2}
o One Reply Path TLV carrying FEC X
o One Reply Path TLV carrying FEC Y
Described encoding of the Reply Mode Order TLV and the Reply Path TLV
in the MPLS echo request message will result in the responder LSR to
prefer "Reply via application level control channel (4)", followed by
FEC X, FEC Y and then "Reply via an IPv4/IPv6 UDP packet (2)".
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4.2.2. Example 2: Reply Mode Order TLV Usage with Reply Path TLV
If the initiator LSR was interested in encoding following return
paths:
1. Reverse LSP
2. Reply via an IPv4/IPv6 UDP packet
3. FEC X
4. FEC Y
Then the MPLS echo request message is to carry:
o The Reply Mode Order TLV carrying Reply Modes {5, 2, 5, 5}
o One Reply Path TLV with the B bit set.
o One Reply Path TLV carrying FEC X
o One Reply Path TLV carrying FEC Y
Described encoding of the Reply Mode Order TLV and the Reply Path TLV
in the MPLS echo request message will result in the responder LSR to
prefer the reverse LSP, followed by "Reply via an IPv4/IPv6 UDP
packet (2)", FEC X and then FEC Y.
4.3. Proxy LSP Ping
The mechanism defined in this document will work with Proxy LSP Ping
defined by [RFC7555]. The MPLS proxy ping request message can carry
a Reply Mode value in the header and one or more Reply Mode values in
the Reply Mode Order TLV. It is RECOMMENDED that the Reply Mode 2
(Reply via an IPv4/IPv6 UDP packet) be used in the Reply Mode field
of the MPLS proxy ping request message.
4.3.1. Proxy LSR Sending an MPLS Echo Request
If the proxy LSR is sending an MPLS echo request, then the proxy LSR
MUST copy the following elements from the MPLS proxy ping request
message to the MPLS echo request message.
o The Reply Mode field.
o The Reply Mode Order TLV.
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o The Reply Path TLV(s). If there are more than one Reply Path
TLVs, then order of them MUST be preserved when copying.
4.3.2. Proxy LSR Sending an MPLS Proxy Ping Reply
If the proxy LSR is sending an MPLS proxy ping reply, then it is
RECOMMENDED that the Reply Mode Order TLV is ignored and the Reply
Mode field in the MPLS proxy ping request message is used.
5. Security Considerations
Those security considerations specified in [RFC4379] and [RFC7110]
apply for this document.
In addition, this document introduces the Reply Mode Order TLV. It
provides a new way for an unauthorized source to gather more network
information, especially the potential return path(s) information of
an LSP. To protect against unauthorized sources using MPLS echo
request messages with the Reply Mode Order TLV to obtain network
information, similar to [RFC4379], it is RECOMMENDED that
implementations provide a means of checking the source addresses of
MPLS echo request messages against an access list before accepting
the message.
Another potential security issue is that the MPLS echo request and
reply messages are not encrypted, the content of the MPLS echo
request and reply messages may be potentially exposed. Although the
exposure is within the MPLS domain, if such exposure is a concern,
some encryption mechanisms [I-D.ietf-mpls-opportunistic-encrypt] may
be employed.
6. Manageability Considerations
Section 2 described the problems which increases the complexity with
respect to operations and implementations. In order to simplify
operations and to allow for the LSP Ping/Traceroute to function
efficiently whilst preserving the code simplicity, it is RECOMMENDED
that implementations allow devices to have configuration options to
set operator preferred Reply Modes. For example:
o For those operators who are more interested in MPLS echo reply
packets reaching back to the initiator LSR:
1. Reply via an IPv4/IPv6 UDP packet (2)
2. Reply via application level control channel (4)
3. Reply via Specified Path (5)
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o For those operators who are more interested in MPLS echo reply
packets testing the paths related to the forward LSP:
1. Reply via Specified Path (5)
2. Reply via application level control channel (4)
3. Reply via an IPv4/IPv6 UDP packet (2)
7. IANA Considerations
7.1. New Reply Mode Order TLV
IANA is requested to assign a new TLV type value from the "TLVs" sub-
registry within the "Multiprotocol Label Switching Architecture
(MPLS)" registry, for the "Reply Mode Order TLV".
The new TLV Type value should be assigned from the range
(32768-49161) specified in [RFC4379] section 3 that allows the TLV
type to be silently dropped if not recognized.
Type Meaning Reference
---- ------- ---------
TBD1 Reply Mode Order TLV this document
8. Acknowledgements
Authors would like to thank Santiago Alvarez and Faisal Iqbal for
discussions which motivated creation of this document. Authors would
also like to thank Sam Aldrin, Curtis Villamizar, Ross Callon,
Jeffrey Zhang, Jeremy Whittaker, Mustapha Alissaoui, Qin Wu, Jie Dong
and Adrian Farrel for providing valuable comments to influence the
contents of the draft. Authors would also like to thank Dan Frost,
Tom Taylor, Victor Kuarsingh and Deborah Brungard for reviewing the
document and providing useful comments.
9. Contributing Authors
Shaleen Saxena
Brocade
Email: ssaxena@brocade.com
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.
[RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
"Return Path Specified Label Switched Path (LSP) Ping",
RFC 7110, DOI 10.17487/RFC7110, January 2014,
<http://www.rfc-editor.org/info/rfc7110>.
10.2. Informative References
[I-D.ietf-mpls-opportunistic-encrypt]
Farrel, A. and S. Farrell, "Opportunistic Security in MPLS
Networks", draft-ietf-mpls-opportunistic-encrypt-00 (work
in progress), July 2015.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
DOI 10.17487/RFC3473, January 2003,
<http://www.rfc-editor.org/info/rfc3473>.
[RFC5960] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
Transport Profile Data Plane Architecture", RFC 5960,
DOI 10.17487/RFC5960, August 2010,
<http://www.rfc-editor.org/info/rfc5960>.
[RFC7555] Swallow, G., Lim, V., and S. Aldrin, "Proxy MPLS Echo
Request", RFC 7555, DOI 10.17487/RFC7555, June 2015,
<http://www.rfc-editor.org/info/rfc7555>.
Appendix A. Reply Mode Order TLV Beneficial Scenarios
This section lists examples of how the Reply Mode Order TLV can
benefit.
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A.1. Incorrect Forwarding Scenario
As shown in Figure 2, a network has an LSP with the forwarding path:
A-B-C-D-E. The LSP has a control channel.
A------B------C------D------E
|
|
F
Forward Paths: A-B-C-D-E
Figure 2: Incorrect Forwarding
If D is incorrectly label switching to F (instead of E). In this
scenario, LSP Traceroute with "Reply via application level control
channel (4)" will result in following result.
Success (Reply from B)
Success (Reply from C)
Success (Reply from D)
Timeout...
Complete
This is because F does not have a control channel to send the MPLS
echo reply message. With the extension described in this document,
same procedures can be performed with the Reply Mode Order TLV
carrying {4, 2}. When LSP Traceroute is issued, then following output
may be displayed without any unnecessary timeout.
Success (Reply from B, Reply Mode: 4)
Success (Reply from C, Reply Mode: 4)
Success (Reply from D, Reply Mode: 4)
FEC Mismatch (Reply from F, Reply Mode: 2)
Complete
The result provides more diagnostic information to the initiator LSR,
and without any delay (i.e. timeout from one or more downstream
LSRs).
A.2. Non-Co-Routed Bidirectional LSP Scenario
As shown in Figure 3, a network has a bidirectional LSP where the
forward LSP and the reverse LSP are not fully co-routed.
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+----C------D----+
/ \
A------B G------H
\ /
+----E------F----+
Forward Paths: A-B-C-D-G-H (upper path)
Reverse Paths: H-G-F-E-B-A (lower path)
Figure 3: Non-Co-Routed Bidirectional LSP
Some operators may prefer and configure the system to default the
Reply Mode to indicate the reverse LSP when MPLS echo request
messages are sent on bidirectional LSPs. Without extensions
described in this document, following behaviors will be seen:
o When LSP Ping is issued from A, the reply will come back on the
reverse LSP from H.
o When LSP Traceroute is issued from A, the replies will come back
on the reverse LSP from B, G and H, but will encounter a timeout
from C and D as there are no reverse LSP on those nodes.
o When LSP Ping with specific TTL value is issued from A, whether a
timeout will be encountered depends on the value of the TTL used
(i.e. whether or not the MPLS echo request terminates on a node
that has reverse LSP).
One can argue that the initiator LSR can automatically generate the
same MPLS echo request with different Reply Mode value to those nodes
that timeout. However, such mechanism will result in extended time
for the entire operation to complete (i.e. multiple seconds to
multiple minutes). This is undesirable, and perhaps unacceptable if
the "user" is an application.
With the extension described in this document, same procedures can be
performed with the Reply Mode Order TLV carrying {5, 2}. When LSP
Traceroute is issued, then following output may be displayed without
any unnecessary timeout.
Success (Reply Mode: 5)
Success (Reply Mode: 2)
Success (Reply Mode: 2)
Success (Reply Mode: 5)
Success (Reply Mode: 5)
Complete
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Authors' Addresses
Nobo Akiya
Big Switch Networks
Email: nobo.akiya.dev@gmail.com
George Swallow
Cisco Systems
Email: swallow@cisco.com
Carlos Pignataro
Cisco Systems
Email: cpignata@cisco.com
Loa Andersson
Huawei
Email: loa@mail01.huawei.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
Akiya, et al. Expires April 10, 2016 [Page 16]
