Internet DRAFT - draft-akiya-mpls-lsp-ping-reply-mode-simple
draft-akiya-mpls-lsp-ping-reply-mode-simple
Internet Engineering Task Force N. Akiya
Internet-Draft G. Swallow
Updates: 4379 (if approved) C. Pignataro
Intended status: Standards Track Cisco Systems
Expires: January 28, 2015 L. Andersson
M. Chen
Huawei
July 27, 2014
Label Switched Path (LSP) Ping/Traceroute Reply Mode Simplification
draft-akiya-mpls-lsp-ping-reply-mode-simple-03
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 adds one value to the
Reply Mode field to indicate reverse LSP. This document also adds an
optional TLV which can carry ordered list of Reply Mode values.
This document updates RFC4379.
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."
This Internet-Draft will expire on January 28, 2015.
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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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statements . . . . . . . . . . . . . . . . . . . . . 3
3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Reply via reverse LSP . . . . . . . . . . . . . . . . . . 5
3.2. Reply Mode Order TLV . . . . . . . . . . . . . . . . . . 5
4. Relations to Other LSP Ping/Trace Features . . . . . . . . . 6
4.1. Reply Path TLV . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Reply Mode Order TLV Usage Example with Reply Path
TLV . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Proxy LSP Ping . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. New Reply Mode . . . . . . . . . . . . . . . . . . . . . 8
6.2. New Reply Mode Order TLV . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Reply Mode Order TLV Beneficial Scenarios . . . . . 9
A.1. Incorrect Forwarding Scenario . . . . . . . . . . . . . . 9
A.2. Non-Co-Routed Bidirectional LSP Scenario . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The MPLS LSP Ping, described in [RFC4379], allows an initiator to
encode instructions (Reply Mode) on how a responder should send the
response back to the initiator. [RFC7110] also allows the initiator
to encode a TLV (Reply Path TLV) which can instruct the responder to
use specific LSP to send the response back to the initiator. Both
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approaches are powerful as they provide the ability for the initiator
to control the return path.
However, it is becoming increasingly difficult for an initiator to
select a valid return path to encode in the MPLS LSP echo request
packets. If the initiator does not select a valid return path, the
MPLS LSP echo reply will not get back to the initiator. 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 further describes the problem in Section 2, and
proposes a solution in Section 3 to minimizes false failure scenarios
while accommodating operator preferences. Additionally, Appendix A
provides examples of scenarios where the mechanism described in this
document provides benefits.
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.
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 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 LSP is completely co-routed, partially co-routed or
associated (i.e., 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 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.
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Except for the case where the responder node does not have an IP
route back to the initiator, it is possible to use Reply Mode of
value 2 (Reply via an IPv4/IPv6 UDP packet) 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 node receives an MPLS echo request containing return path
instructions which cannot be accommodated due to unavailability, then
the responder often drops such packets. This results in the
initiator not receiving the MPLS LSP echo reply packets back. This
consequence may be acceptable for failure cases (e.g., broken LSPs)
where the MPLS echo request terminated on unintended target.
However, the initiator not receiving back MPLS echo reply packets,
even when the intended target received and verified the requests, is
not desirable as false failures will be conveyed to users.
Many operators prefer some return path(s) over others for specific
LSP types. To accommodate this, implementations may default to
operator preferred return path (or allow default return path to be
configured) for a specific operation. However, if the sender of MPLS
echo request knew that preferred return path will not be available at
the intended target node, then it is not very beneficial to use a
Reply Mode corresponding to preferred return path (i.e., the sender
of the MPLS echo request will not receive the MPLS echo reply in the
successful case). What would be beneficial, for a given operation,
is for the sender of the MPLS echo request to determine which return
path(s) can and cannot be used ahead of time.
This document adds one Reply Mode value to describe the reverse LSP,
and 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 to use the first
available Reply Mode from the list. This eliminates the need for the
initiator to compute, or sometimes guess, the default return path
encoding. And that will result in simplified implementations across
vendors, and result in improved usability to fit operational needs.
3. Solution
This document adds one reply mode to indicate reverse LSP, to be used
by the MPLS LSP Ping and Traceroute. This document also adds an
optional TLV which can carry ordered list of reply modes.
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3.1. Reply via reverse LSP
Some LSP types are capable of having related LSP in reverse
direction, through signaling or other association mechanisms.
Examples of such LSP types are RSVP LSPs and TP LSPs. This document
uses the term "Reverse LSP" to refer to the LSP in 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.
This document adds one Reply Mode to be used by MPLS LSP Ping and
Traceroute operations.
Value Meaning
----- -------
TBD1 Reply via reverse LSP
MPLS echo request with TBD1 (Reply via reverse LSP) in the Reply Mode
field may be used to instruct responder to use reverse LSP to send
MPLS echo reply. Reverse LSP is in relation to the last FEC
specified in the Target FEC Stack TLV.
When a responder is using this Reply Mode, transmitting MPLS echo
reply packet MUST use IP destination address of 127/8 for IPv4 and
0:0:0:0:0:FFFF:7F00/104 for IPv6.
3.2. Reply Mode Order TLV
This document also introduces a new optional TLV to describe 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. Reply Mode Order TLV MAY be included in MPLS echo request.
2. Reply Mode Order TLV MUST NOT be included in MPLS echo reply.
3. Reply Mode field of MPLS echo request MUST be set to a valid
value when supplying Reply Mode Order TLV in transmitting MPLS
echo request. The initiator SHOULD set Reply Mode field of MPLS
echo request to a value that corresponds to a return path which
most likely to be available, in case responder does not
understand the Reply Mode Order TLV.
4. If a responder node understands the Reply Mode Order TLV and the
TLV is valid, then the responder MUST consider Reply Mode values
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described in the TLV and MUST NOT use the value described in the
Reply Mode field of received MPLS echo request.
5. If a responder node understands the Reply Mode Order TLV but the
TLV is not valid (due to conditions listed below), then the
responder MUST only use the value described in the Reply Mode
field of received MPLS echo request.
6. Reply Mode Order TLV MUST contain at least one Reply Mode value,
and SHOULD contain at least two Reply Mode values.
7. A Reply Mode value MUST NOT be repeated (i.e. MUST NOT appear
multiple times) in the Reply Mode Order TLV.
8. Reply Mode value 1 (Do not reply) SHOULD NOT be used in the Reply
Mode Order TLV.
The responding node is to select the first available return path in
this TLV. Reply Mode value corresponding to selected return path
MUST be set in Reply Mode field of MPLS echo reply to communicate
back to the initiator which return path was chosen.
The format of the TLV is as follows:
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. Each Reply Mode field is 1
octet.
4. Relations to Other LSP Ping/Trace Features
4.1. Reply Path TLV
[RFC7110] defines a new Reply Mode (5 - Reply via Specified Path).
This Reply Mode specified in MPLS echo request indicates that MPLS
echo reply be sent on one specific path described by the Reply Path
TLV. The Flags field of the Reply Path TLV can indicate B
(Bidirectional) bit to describe reverse direction of the tested
bidirectional LSP. However, it is desired to have a new Reply Mode
(TBD1 - Reply via reverse LSP) to indicate reverse direction of the
tested bidirectional LSP without requiring to include additional TLV
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(i.e. Reply Path TLV). Therefore, a new Reply Mode (TBD1 - Reply
via reverse LSP) has been added.
4.1.1. Reply Mode Order TLV Usage Example with Reply Path TLV
If the initiator 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, 2}
o The Reply Path TLV carrying {FEC X, 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 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)".
4.2. Proxy LSP Ping
The mechanism defined in this document will work with Proxy LSP Ping
defined by [I-D.ietf-mpls-proxy-lsp-ping]. MPLS proxy ping request
can carry a Reply Mode value and the Reply Mode Order TLV with list
of Reply Mode values. Proxy LSR MUST copy both Reply Mode value and
the Reply Mode Order TLV into MPLS echo request. Proxy LSR, upon
receiving MPLS echo reply, MUST copy Reply Mode value into MPLS proxy
ping reply. With these procedures, Reply Mode used by the MPLS echo
reply sender is propagated in the Reply Mode field to the sender of
MPLS proxy ping request.
5. Security Considerations
Beyond those specified in [RFC4379], there are no further security
measures required.
6. IANA Considerations
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6.1. New Reply Mode
IANA is requested to assign one reply modes from the "Reply Mode"
sub-registry within the "Multiprotocol Label Switching Architecture
(MPLS)" registry.
Value Meaning Reference
----- ------- ---------
TBD1 Reply via reverse LSP this document
6.2. 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
---- ------- ---------
TBD2 Reply Mode Order TLV this document
7. 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 and Mustapha Alissaoui for providing
valuable comments to influence the contents of the draft.
8. Contributing Authors
Shaleen Saxena
Cisco Systems
Email: ssaxena@cisco.com
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
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9.2. Informative References
[I-D.ietf-mpls-proxy-lsp-ping]
Swallow, G., Lim, V., and S. Aldrin, "Proxy MPLS Echo
Request", draft-ietf-mpls-proxy-lsp-ping-02 (work in
progress), July 2014.
[RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
"Return Path Specified Label Switched Path (LSP) Ping",
RFC 7110, January 2014.
Appendix A. Reply Mode Order TLV Beneficial Scenarios
This section lists examples of how the Reply Mode Order TLV can
benefit.
A.1. Incorrect Forwarding Scenario
A network has a following LSP, and the LSP has a control channel.
A------B------C------D------E
|
|
F
Forward Paths: A-B-C-D-E
Figure 2: Incorrect Forwarding
Imagine that 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)
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FEC Mismatch (Reply from F, Reply Mode: 2)
Complete
The result provides more diagnostic information to the initiator, and
without any delay (i.e. timeout from one or more downstream LSRs).
A.2. Non-Co-Routed Bidirectional LSP Scenario
A network has a following bidirectional LSP where the forward LSP and
the reverse LSP are not fully co-routed.
+----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 "Reply via reverse LSP (TBD1)" 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, reply will come back on the
reverse LSP from H.
o When LSP Traceroute is issued from A, reply 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 MPLS echo request terminates on a node that
has reverse LSP).
One can argue that the initiator can automatically generate a 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 {TBD1, 2}. When LSP
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Traceroute is issued, then following output may be displayed without
any unnecessary timeout.
Success (Reply Mode: TBD1)
Success (Reply Mode: 2)
Success (Reply Mode: 2)
Success (Reply Mode: TBD1)
Success (Reply Mode: TBD1)
Complete
Authors' Addresses
Nobo Akiya
Cisco Systems
Email: nobo@cisco.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
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