Internet Engineering Task Force (IETF) N. Akiya
Request for Comments: 8012 Big Switch Networks
Updates: 6790 G. Swallow
Category: Standards Track C. Pignataro
ISSN: 2070-1721 Cisco
A. Malis
Huawei Technologies
S. Aldrin
Google
November 2016
Label Switched Path (LSP) and Pseudowire (PW) Ping/Trace
over MPLS Networks Using Entropy Labels (ELs)
Abstract
Multiprotocol Label Switching (MPLS) Label Switched Path (LSP) ping
and traceroute are methods used to test Equal-Cost Multipath (ECMP)
paths. Ping is known as a connectivity-verification method and
traceroute is known as a fault-isolation method, as described in RFC
4379. When an LSP is signaled using the Entropy Label (EL) described
in RFC 6790, the ability for LSP ping and traceroute operations to
discover and exercise ECMP paths is lost for scenarios where Label
Switching Routers (LSRs) apply different load-balancing techniques.
One such scenario is when some LSRs apply EL-based load balancing
while other LSRs apply load balancing that is not EL based (e.g.,
IP). Another scenario is when an EL-based LSP is stitched with
another LSP that can be EL based or not EL based.
This document extends the MPLS LSP ping and traceroute multipath
mechanisms in RFC 6424 to allow the ability of exercising LSPs that
make use of the EL. This document updates RFC 6790.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8012.
Copyright Notice
Copyright (c) 2016 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
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 ....................................................3
1.1. Terminology ................................................5
1.1.1. Requirements Language ...............................6
1.2. Background .................................................6
2. Multipath Type {9} ..............................................7
3. Pseudowire Tracing ..............................................7
4. Entropy Label FEC ...............................................8
5. DS Flags: L and E ...............................................9
6. New Multipath Information Type {10} ............................10
7. Initiating LSR Procedures ......................................12
8. Responder LSR Procedures .......................................14
8.1. IP-Based Load Balancer That Does Not Push ELI/EL ..........15
8.2. IP-Based Load Balancer That Pushes ELI/EL .................15
8.3. Label-Based Load Balancer That Does Not Push ELI/EL .......16
8.4. Label-Based Load Balancer That Pushes ELI/EL ..............17
8.5. Flow-Aware MS-PW Stitching LSR ............................18
9. Supported and Unsupported Cases ................................18
10. Security Considerations .......................................20
11. IANA Considerations ...........................................21
11.1. Entropy Label FEC ........................................21
11.2. DS Flags .................................................21
11.3. Multipath Type ...........................................21
12. References ....................................................22
12.1. Normative References .....................................22
12.2. Informative References ...................................22
Acknowledgements ..................................................23
Contributors ......................................................23
Authors' Addresses ................................................23
1. Introduction
[RFC4379] describes LSP traceroute as an operation where the
initiating LSR sends a series of MPLS echo requests towards the same
destination. The first packet in the series has the TTL set to 1.
When the echo reply is received from the LSR one hop away, the second
echo request in the series is sent with the TTL set to 2. For each
additional echo request, the TTL is incremented by one until a
response is received from the intended destination. The initiating
LSR discovers and exercises ECMP by obtaining Multipath Information
from each transit LSR and using a specific destination IP address or
specific entropy label.
From here on, the notation {x, y, z} refers to Multipath Information
Types x, y, or z. Multipath Information Types are defined in
Section 3.3 of [RFC4379] .
The LSR initiating LSP ping sends an MPLS echo request with the
Multipath Information. This Multipath Information is described in
the echo request's DDMAP TLV and may contain a set of IP addresses or
a set of labels. Multipath Information Types {2, 4, 8} carry a set
of IP addresses, and the Multipath Information Type {9} carries a set
of labels. The responder LSR (the receiver of the MPLS echo request)
will determine the subset of initiator-specified Multipath
Information, which load balances to each downstream (outgoing)
interface. The responder LSR sends an MPLS echo reply with the
resulting Multipath Information per downstream (outgoing interface)
back to the initiating LSR. The initiating LSR is then able to use a
specific IP destination address or a specific label to exercise a
specific ECMP path on the responder LSR.
The current behavior is problematic in the following scenarios:
o The initiating LSR sends the IP Multipath Information, but the
responder LSR load balances on labels.
o The initiating LSR sends the Label Multipath Information, but the
responder LSR load balances on IP addresses.
o The initiating LSR sends the existing Multipath Information to an
LSR that pushes ELI/EL in the label stack, but the initiating LSR
can only continue to discover and exercise specific paths of the
ECMP if the LSR that pushes ELI/EL responds with both IP addresses
and the associated EL corresponding to each IP address. This is
because:
* An ELI/EL-pushing LSR that is a stitching point will load
balance based on the IP address.
* Downstream LSR(s) of an ELI/EL-pushing LSR may load balance
based on ELs.
o The initiating LSR sends existing Multipath Information to an ELI/
EL-pushing LSR, but the initiating LSR can only continue to
discover and exercise specific paths of ECMP if the ELI/EL-pushing
LSR responds with both labels and the associated EL corresponding
to the label. This is because:
* An ELI/EL-pushing LSR that is a stitching point will load
balance based on the EL from the previous LSP and push a new
EL.
* Downstream LSR(s) of ELI/EL-pushing LSR may load balance based
on new ELs.
The above scenarios demonstrate that the existing Multipath
Information is insufficient when LSP traceroute is used on an LSP
with entropy labels [RFC6790]. This document defines a new Multipath
Information Type to be used in the DDMAP of MPLS echo request/reply
packets for [RFC6790] LSPs.
The responder LSR can reply with empty Multipath Information if no IP
address set or if no label set is received with the Multipath
Information. An empty return is also possible if an initiating LSR
sends Multipath Information of one type, IP Address or Label, but the
responder LSR load balances on the other type. To disambiguate
between the two results, this document introduces new flags in the
DDMAP TLV to allow the responder LSR to describe the load-balancing
technique being used.
To use this enhanced method end-to-end, all LSRs along the LSP need
to be able to understand the new flags and the new Multipath
Information Type. Mechanisms to verify this condition are outside of
the scope of this document. The rest of the requirements are
detailed in the initiating LSR and responder LSR procedures. Two
additional DS Flags are defined for the DDMAP TLV in Section 6.
These two flags are used by the responder LSR to describe its load-
balancing behavior on a received MPLS echo request.
Note that the terms "IP-Based Load Balancer" and "Label-Based Load
Balancer" are in context of how a received MPLS echo request is
handled by the responder LSR.
1.1. Terminology
The following abbreviations and terms are used in this document:
o MPLS: Multiprotocol Label Switching.
o LSP: Label Switched Path.
o Stitched LSP: Stitched Label Switched Paths combine several LSPs
such that a single end-to-end LSP is realized. [RFC6424]
describes LSP ping for Stitched LSPs.
o LSR: Label Switching Router.
o FEC: Forwarding Equivalence Class.
o ECMP: Equal-Cost Multipath.
o EL: Entropy Label.
o ELI: Entropy Label Indicator.
o GAL: Generic Associated Channel Label.
o MS-PW: Multi-Segment Pseudowire.
o Initiating LSR: An LSR that sends an MPLS echo request.
o Responder LSR: An LSR that receives an MPLS echo request and sends
an MPLS echo reply.
o IP-Based Load Balancer: An LSR that load balances on fields from
an IP header (and possibly fields from upper layers) and does not
consider an entropy label from an MPLS label stack (i.e., flow
label [RFC6391] or entropy label [RFC6790]) for load-balancing
purposes.
o Label-Based Load Balancer: An LSR that load balances on an entropy
label from an MPLS label stack (i.e., flow label or entropy label)
and does not consider fields from an IP header (and possibly
fields from upper layers) for load-balancing purposes.
o Label and IP-Based Load Balancer: An LSR that load balances on
both entropy labels from an MPLS label stack and fields from an IP
header (and possibly fields from upper layers).
1.1.1. 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].
1.2. Background
MPLS implementations employ a wide variety of load-balancing
techniques in terms of fields used for hash "keys". The mechanisms
in [RFC4379] and updated by [RFC6424] are designed to provide
multipath support for a subset of techniques. The intent of this
document is to provide multipath support for the supported techniques
that are compromised by the use of ELs [RFC6790]. Section 9
describes supported and unsupported cases, and it may be useful for
the reader to first review this section.
The Downstream Detailed Mapping (DDMAP) TLV [RFC6424] provides
Multipath Information, which can be used by an LSP ping initiator to
trace and validate ECMP paths between an ingress and egress. The
Multipath Information encodings defined by [RFC6424] are sufficient
when all the LSRs along the path(s), between ingress and egress,
consider the same set of "keys" as input for load-balancing
algorithms, e.g., either all IP based or all label based.
With the introduction of [RFC6790], some LSRs may perform load
balancing based on labels while others may be IP based. This results
in an LSP ping initiator that is unable to trace and validate all the
ECMP paths in the following scenarios:
o One or more transit LSRs along an LSP with ELI/EL in the label
stack do not perform ECMP load balancing based on EL (hashes based
on "keys" including the IP destination address). This scenario is
not only possible but quite common due to transit LSRs not
implementing [RFC6790] or transit LSRs implementing [RFC6790] but
not implementing the suggested transit LSR behavior in Section 4.3
of [RFC6790].
o Two or more LSPs stitched together with at least one of these LSPs
pushing ELI/EL into the label stack.
These scenarios can be quite common because deployments of [RFC6790]
typically have a mixture of nodes that support ELI/EL and nodes that
do not. There will also typically be a mixture of areas that support
ELI/EL and areas that do not.
As pointed out in [RFC6790], the procedures of [RFC4379] (and
consequently of [RFC6424]) with respect to Multipath Information Type
{9} are incomplete. However, [RFC6790] does not actually update
[RFC4379]. Further, the specific EL location is not clearly defined,
particularly in the case of Flow-Aware Pseudowires [RFC6391]. This
document defines a new FEC Stack sub-TLV for the entropy label.
Section 2 of this document updates the procedures for the Multipath
Information Type {9} that are described in [RFC4379] and that are
applicable to [RFC6424]. The rest of this document describes
extensions required to restore ECMP discovery and tracing
capabilities for the scenarios described.
[RFC4379], [RFC6424], and this document will support IP-based load
balancers and label-based load balancers that limit their hash to the
first (top-most) or only entropy label in the label stack. Other use
cases (refer to Section 9) are out of scope.
2. Multipath Type {9}
[RFC4379] defined Multipath Type {9} for the tracing of LSPs where
label-based load balancing is used. However, as pointed out in
[RFC6790], the procedures for using this type are incomplete as the
specific location of the label was not defined. It was assumed that
the presence of Multipath Type {9} implied that the value of the
bottom-of-stack label should be varied by the values indicated by the
multipath to determine the respective outgoing interfaces.
Section 4 defines a new FEC-Stack sub-TLV to indicate an entropy
label. These labels MAY appear anywhere in a label stack.
Multipath Type {9} applies to the first label in the label stack that
corresponds to an EL-FEC. If no such label is found, it applies to
the label at the bottom of the label stack.
3. Pseudowire Tracing
This section defines procedures for tracing Pseudowires. These
procedures pertain to the use of Multipath Information Type {9} as
well as Type {10}. In all cases below, when a control word is in
use, the N flag in the DDMAP MUST be set. Note that when a control
word is not in use, the returned DDMAPs may not be accurate.
In order to trace a Pseudowire that is not flow aware, the initiator
includes an EL-FEC instead of the appropriate PW FEC at the bottom of
the FEC Stack. Tracing in this way will cause compliant routers to
return the proper outgoing interface. Note that this procedure only
traces to the end of the MPLS LSP that is under test and will not
verify the PW FEC. To actually verify the PW FEC or in the case of a
MS-PW, to determine the next Pseudowire label value, the initiator
MUST repeat that step of the trace (i.e., repeating the TTL value
used) but with the FEC Stack modified to contain the appropriate PW
FEC. Note that these procedures are applicable to scenarios where an
initiator is able to vary the bottom label (i.e., Pseudowire label).
Possible scenarios are tracing multiple Pseudowires that are not flow
aware on the same endpoints or tracing a Pseudowire that is not flow-
aware provisioned with multiple Pseudowire labels.
In order to trace a flow-aware Pseudowire [RFC6391], the initiator
includes an EL FEC at the bottom of the FEC Stack and pushes the
appropriate PW FEC onto the FEC Stack.
In order to trace through routers that are not compliant, the
initiator forms an MPLS echo request message and includes a DDMAP
with the Multipath Type {9}. For a Pseudowire that is not flow
aware, it includes the appropriate PW FEC in the FEC Stack. For a
flow- aware Pseudowire, the initiator includes a Nil FEC at the
bottom of the FEC Stack and pushes the appropriate PW FEC onto the
FEC Stack.
4. Entropy Label FEC
The ELI is a reserved label that has no associated explicit FEC, and
has the label value 7 assigned from the reserved range. Use the Nil
FEC as the Target FEC Stack sub-TLV to account for ELI in a Target
FEC Stack TLV.
The EL is a special-purpose label with the label value being
discretionary (i.e., the label value is not from the reserved range).
For LSP verification mechanics to perform its purpose, it is
necessary for a Target FEC Stack sub-TLV to clearly describe the EL,
particularly in the scenario where the label stack does not carry ELI
(e.g., flow-aware Pseudowire [RFC6391]). Therefore, this document
defines an EL FEC sub-TLV (33, see Section 11.1) that allows a Target
FEC Stack sub-TLV to be added to the Target FEC Stack to account for
EL.
The Length is 4. Labels are 20-bit values treated as numbers.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Entropy Label FEC
"Label" is the actual label value inserted in the label stack; the
"MBZ" field MUST be zero when sent and ignored on receipt.
5. DS Flags: L and E
Two flags, L and E, are added to the DS Flags field of the DDMAP TLV.
Both flags MUST NOT be set in the echo request packets when sending
and SHOULD be ignored when received. Zero, one, or both new flags
MUST be set in the echo reply packets.
DS Flags
--------
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |L|E|I|N|
+-+-+-+-+-+-+-+-+
Flag Name and Meaning
---- ----------------
L Label-based load balance indicator
This flag MUST be cleared in the echo request. An LSR
that performs load balancing on a label MUST set this
flag in the echo reply. An LSR that performs load
balancing on IP MUST NOT set this flag in the echo
reply.
E ELI/EL push indicator
This flag MUST be cleared in the echo request. An LSR
that pushes ELI/EL MUST set this flag in the echo
reply. An LSR that does not push ELI/EL MUST NOT set
this flag in the echo reply.
The two flags result in four load-balancing techniques, which the
echo reply generating LSR can indicate:
o {L=0, E=0} LSR load balances based on IP and does not push ELI/EL.
o {L=0, E=1} LSR load balances based on IP and pushes ELI/EL.
o {L=1, E=0} LSR load balances based on labels and does not push
ELI/EL.
o {L=1, E=1} LSR load balances based on labels and pushes ELI/EL.
6. New Multipath Information Type {10}
One new Multipath Information Type is added to be used in DDMAP TLV.
This new Multipath Type has the value of 10.
Key Type Multipath Information
--- ---------------- ---------------------
10 IP and Label set IP addresses and label prefixes
Multipath Information Type {10} is comprised of three sections. The
first section describes the IP address set. The second section
describes the label set. The third section describes another label
set, which associates to either the IP address set or the label set
specified in the other sections.
Multipath Information Type {10} 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPMultipathType| IP Multipath Length | Reserved(MBZ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (IP Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LbMultipathType| Label Multipath Length | Reserved(MBZ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (Label Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assoc. Label Multipath Length | Reserved(MBZ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (Associated Label Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Multipath Information Type {10}
o IPMultipathType
* 0 when "IP Multipath Information" is omitted. Otherwise, one
of the IP Multipath Information values: {2, 4, 8}.
o IP Multipath Information
* This section is omitted when "IPMultipathType" is 0.
Otherwise, this section reuses the IP Multipath Information
from [RFC4379]. Specifically, Multipath Information for values
{2, 4, 8} can be used.
o LbMultipathType
* 0 when the "Label Multipath Information" is omitted.
Otherwise, the Label Multipath Information value {9}.
o Label Multipath Information
* This section is omitted when the "LbMultipathType" is 0.
Otherwise, this section reuses the Label Multipath Information
from [RFC4379]. Specifically, the Multipath Information for
value {9} can be used.
o Associated Label Multipath Information
* "Associated Label Multipath Length" is a 16-bit field of
Multipath Information that indicates the length in octets of
the Associated Label Multipath Information.
* "Associated Label Multipath Information" is a list of labels
with each label described in 24 bits. This section MUST be
omitted in an MPLS echo request message. A midpoint that
pushes ELI/EL labels SHOULD include "Associated Label Multipath
Information" in its MPLS echo reply message, along with either
"IP Multipath Information" or "Label Multipath Information".
Each specified associated label described in this section maps
to a specific IP address OR label described in the "IP
Multipath Information" section or the "Label Multipath
Information" section. For example, if three IP addresses are
specified in the "IP Multipath Information" section, then there
MUST be three labels described in this section. The first
label maps to the first IP address specified, the second label
maps to the second IP address specified, and the third label
maps to the third IP address specified.
When a section is omitted, the length for that section MUST be set to
zero.
7. Initiating LSR Procedures
The following procedure is described in terms of an EL_LSP boolean
maintained by the initiating LSR. This value controls the Multipath
Information Type to be used in the transmitted echo request packets.
When the initiating LSR is transmitting an echo request packet with
DDMAP with a non-zero Multipath Information Type, then the EL_LSP
boolean MUST be consulted to determine the Multipath Information Type
to use.
In addition to the procedures described in [RFC4379], as updated by
Section 2 and [RFC6424], the initiating LSR MUST operate with the
following procedures:
o When the initiating LSR pushes ELI/EL, initialize EL_LSP=True.
Else, set EL_LSP=False.
o When the initiating LSR is transmitting a non-zero Multipath
Information Type:
* If (EL_LSP), the initiating LSR MUST use the Multipath
Information Type {10} unless the responder LSR cannot handle
Type {10}. When the initiating LSR is transmitting the
Multipath Information Type {10}, both "IP Multipath
Information" and "Label Multipath Information" MUST be
included, and "Associated Label Multipath Information" MUST be
omitted (NULL).
* Else, the initiating LSR MAY use the Multipath Information Type
{2, 4, 8, 9, 10}. When the initiating LSR is transmitting the
Multipath Information Type {10} in this case, "IP Multipath
Information" MUST be included, and "Label Multipath
Information" and "Associated Label Multipath Information" MUST
be omitted (NULL).
o When the initiating LSR receives an echo reply with {L=0, E=1} in
the DS Flags with valid contents, set EL_LSP=True.
In the following conditions, the initiating LSR may have lost the
ability to exercise specific ECMP paths. The initiating LSR MAY
continue with "best effort" in the following cases:
o Received echo reply contains empty Multipath Information.
o Received echo reply contains {L=0, E=<any>} DS Flags, but does not
contain IP Multipath Information.
o Received echo reply contains {L=1, E=<any>} DS Flags, but does not
contain Label Multipath Information.
o Received echo reply contains {L=<any>, E=1} DS Flags, but does not
contain Associated Label Multipath Information.
o IP Multipath Information Types {2, 4, 8} sent, and received echo
reply with {L=1, E=0} in DS Flags.
o Multipath Information Type {10} sent, and received echo reply with
Multipath Information Type other than {10}.
8. Responder LSR Procedures
Common Procedures:
o The responder LSR receiving an MPLS echo request packet MUST first
determine whether or not the initiating LSR supports this LSP ping
and traceroute extension for entropy labels. If either of the
following conditions are met, the responder LSR SHOULD determine
that the initiating LSR supports this LSP ping and traceroute
extension for entropy labels.
1. Received MPLS echo request contains the Multipath Information
Type {10}.
2. Received MPLS echo request contains a Target FEC Stack TLV
that includes the entropy label FEC.
If the initiating LSR is determined not to support this LSP ping
and traceroute extension for entropy labels, then the responder
LSR MUST NOT follow further procedures described in this section.
Specifically, MPLS echo reply packets:
* MUST have the following DS Flags cleared (i.e., not set): "ELI/
EL push indicator" and "Label-based load balance indicator".
* MUST NOT use the Multipath Information Type {10}.
o The responder LSR receiving an MPLS echo request packet with the
Multipath Information Type {10} MUST validate the following
contents. Any deviation MUST result in the responder LSR
considering the packet to be malformed and returning code 1
("Malformed echo request received") in the MPLS echo reply packet.
* IP Multipath Information MUST be included.
* Label Multipath Information MAY be included.
* Associated Label Multipath Information MUST be omitted (NULL).
The following subsections describe expected responder LSR procedures
when the echo reply is to include DDMAP TLVs, based on the local load
balance technique being employed. In case the responder LSR performs
deviating load balance techniques on a per-downstream basis,
appropriate procedures matched to each downstream load balance
technique MUST be followed.
8.1. IP-Based Load Balancer That Does Not Push ELI/EL
o The responder MUST set {L=0, E=0} in DS Flags.
o If the Multipath Information Type {2, 4, 8} is received, the
responder MUST comply with [RFC4379] and [RFC6424].
o If the Multipath Information Type {9} is received, the responder
MUST reply with Multipath Type {0}.
o If the Multipath Information Type {10} is received, the following
procedures are to be used:
* The responder MUST reply with the Multipath Information Type
{10}.
* The "Label Multipath Information" and "Associated Label
Multipath Information" sections MUST be omitted (NULL).
* If no matching IP address is found, then the "IPMultipathType"
field MUST be set to the Multipath Information Type {0} and the
"IP Multipath Information" section MUST also be omitted (NULL).
* If at least one matching IP address is found, then the
"IPMultipathType" field MUST be set to the appropriate
Multipath Information Type {2, 4, 8} and the "IP Multipath
Information" section MUST be included.
8.2. IP-Based Load Balancer That Pushes ELI/EL
o The responder MUST set {L=0, E=1} in DS Flags.
o If the Multipath Information Type {9} is received, the responder
MUST reply with Multipath Type {0}.
o If the Multipath Type {2, 4, 8, 10} is received, the following
procedures are to be used:
* The responder MUST respond with Multipath Type {10}. See
Section 6 for details of Multipath Type {10}.
* The "Label Multipath Information" section MUST be omitted
(i.e., it is not there).
* The IP address set specified in the received IP Multipath
Information MUST be used to determine the returned IP/Label
pairs.
* If the received Multipath Information Type was {10}, the
received "Label Multipath Information" sections MUST NOT be
used to determine the associated label portion of the returned
IP/Label pairs.
* If no matching IP address is found, then the "IPMultipathType"
field MUST be set to the Multipath Information Type {0} and the
"IP Multipath Information" section MUST be omitted. In
addition, the "Associated Label Multipath Length" MUST be set
to 0, and the "Associated Label Multipath Information" section
MUST also be omitted.
* If at least one matching IP address is found, then the
"IPMultipathType" field MUST be set to the appropriate
Multipath Information Type {2, 4, 8} and the "IP Multipath
Information" section MUST be included. In addition, the
"Associated Label Multipath Information" section MUST be
populated with a list of labels corresponding to each IP
address specified in the "IP Multipath Information" section.
"Associated Label Multipath Length" MUST be set to a value
representing the length in octets of the "Associated Label
Multipath Information" field.
8.3. Label-Based Load Balancer That Does Not Push ELI/EL
o The responder MUST set {L=1, E=0} in DS Flags.
o If the Multipath Information Type {2, 4, 8} is received, the
responder MUST reply with Multipath Type {0}.
o If the Multipath Information Type {9} is received, the responder
MUST comply with [RFC4379] and [RFC6424] as updated by Section 2.
o If the Multipath Information Type {10} is received, the following
procedures are to be used:
* The responder MUST reply with the Multipath Information Type
{10}.
* The "IP Multipath Information" and "Associated Label Multipath
Information" sections MUST be omitted (NULL).
* If no matching label is found, then the "LbMultipathType" field
MUST be set to the Multipath Information Type {0} and the
"Label Multipath Information" section MUST also be omitted
(NULL).
* If at least one matching label is found, then the
"LbMultipathType" field MUST be set to the appropriate
Multipath Information Type {9} and the "Label Multipath
Information" section MUST be included.
8.4. Label-Based Load Balancer That Pushes ELI/EL
o The responder MUST set {L=1, E=1} in DS Flags.
o If the Multipath Information Type {2, 4, 8} is received, the
responder MUST reply with Multipath Type {0}.
o If the Multipath Type {9, 10} is received, the following
procedures are to be used:
* The responder MUST respond with the Multipath Type {10}.
* The "IP Multipath Information" section MUST be omitted.
* The label set specified in the received Label Multipath
Information MUST be used to determine the returned Label/Label
pairs.
* If the received Multipath Information Type was {10} received,
the "Label Multipath Information" sections MUST NOT be used to
determine the associated label portion of the returned Label/
Label pairs.
* If no matching label is found, then the "LbMultipathType" field
MUST be set to the Multipath Information Type {0} and the
"Label Multipath Information" section MUST be omitted. In
addition, the "Associated Label Multipath Length" MUST be set
to 0, and the "Associated Label Multipath Information" section
MUST also be omitted.
* If at least one matching label is found, then the
"LbMultipathType" field MUST be set to the appropriate
Multipath Information Type {9} and the "Label Multipath
Information" section MUST be included. In addition, the
"Associated Label Multipath Information" section MUST be
populated with a list of labels corresponding to each label
specified in the "Label Multipath Information" section. The
"Associated Label Multipath Length" MUST be set to a value
representing the length in octets of the "Associated Label
Multipath Information" field.
8.5. Flow-Aware MS-PW Stitching LSR
A stitching LSR that cross-connects flow-aware Pseudowires behaves in
one of two ways:
o Load balances on the previous flow label and carries over the same
flow label. For this case, the stitching LSR is to behave as
described in Section 8.3.
o Load balances on the previous flow label and replaces the flow
label with a newly computed label. For this case, the stitching
LSR is to behave as described in Section 8.4.
9. Supported and Unsupported Cases
The MPLS architecture does not define strict rules on how
implementations are to identify hash "keys" for load-balancing
purposes. As a result, implementations may be of the following load
balancer types:
1. IP-based load balancer.
2. Label-based load balancer.
3. Label- and IP-based load balancer.
For cases (2) and (3), an implementation can include different sets
of labels from the label stack for load-balancing purpose. Thus, the
following sub-cases are possible:
a. Entire label stack.
b. Top N labels from label stack where the number of labels in label
stack is > N.
c. Bottom N labels from label stack where the number of labels in
label stack is > N.
In a scenario where there is one flow label or entropy label present
in the label stack, the following further cases are possible for
(2b), (2c), (3b), and (3c):
1. N labels from label stack include flow label or entropy label.
2. N labels from label stack do not include flow label or entropy
label.
Also, in a scenario where there are multiple entropy labels present
in the label stack, it is possible for implementations to employ
deviating techniques:
o Search for entropy stops at the first entropy label.
o Search for entropy includes any entropy label found plus continues
to search for entropy in the label stack.
Furthermore, handling of reserved (i.e., special) labels varies among
implementations:
o Reserved labels are used in the hash as any other label would be
(not a recommended practice).
o Reserved labels are skipped over and, for implementations limited
to N labels, the reserved labels do not count towards the limit of
N.
o Reserved labels are skipped over and, for implementations limited
to N labels, the reserved labels count towards the limit of N.
It is important to point this out since the presence of GAL will
affect those implementations that include reserved labels for load-
balancing purposes.
As can be seen from the above, there are many types of potential
load-balancing implementations. Attempting to get any Operations,
Administration, and Maintenance (OAM) tools to support ECMP discovery
and traversal over all types would require fairly complex procedures.
Complexities in OAM tools have minimal benefit if the majority of
implementations are expected to employ only a small subset of the
cases described above.
o Section 4.3 of [RFC6790] states that in implementations, for load-
balancing purposes, parsing beyond the label stack after finding
an entropy label has "limited incremental value". Therefore, it
is expected that most implementations will be of types "IP-based
load balancer" or "Label-based load balancer".
o Section 2.4.5.1 of [RFC7325] recommends that searching for entropy
labels in the label stack should terminate upon finding the first
entropy label. Therefore, it is expected that implementations
will only include the first (top-most) entropy label when there
are multiple entropy labels in the label stack.
o It is expected that, in most cases, the number of labels in the
label stack will not exceed the number of labels (N) that
implementations can include for load-balancing purposes.
o It is expected that labels in the label stack, besides the flow
label and entropy label, are constant for the lifetime of a single
LSP multipath traceroute operation. Therefore, deviating load-
balancing implementations with respect to reserved labels should
not affect this tool.
Thus, [RFC4379], [RFC6424], and this document support cases (1) and
(2a1), where only the first (top-most) entropy label is included when
there are multiple entropy labels in the label stack.
10. Security Considerations
While [RFC4379] and [RFC6424] already allow for the discovery and
exercise of ECMP paths, this document extends the LSP ping and
traceroute mechanisms to more precisely discover and exercise ECMP
paths when an LSP uses ELI/EL in the label stack. Sourcing or
inspecting LSP ping packets can be used for network reconnaissance.
The extended capability defined in this document requires minor
additional processing for the responder and initiator nodes. The
responder node that pushes ELI/EL will need to compute and return
multipath data including associated EL. The initiator node will need
to store and handle both IP Multipath and Label Multipath
Information, and include destination IP addresses and/or ELs in MPLS
echo request packets as well as in the Multipath Information sent to
downstream nodes. The security considerations of [RFC4379] already
cover Denial-of-Service attacks by regulating LSP ping traffic going
to the control plane.
Finally, the security measures described in [RFC4379], [RFC6424], and
[RFC6790] are applicable. [RFC6424] provides guidelines if a network
operator wants to prevent tracing or does not want to expose details
of the tunnel and [RFC6790] provides guidance on the use of the EL.
11. IANA Considerations
11.1. Entropy Label FEC
IANA has assigned a new sub-TLV from the "Sub-TLVs for TLV Types 1,
16, and 21" section from the "Multi-Protocol Label Switching (MPLS)
Label Switched Paths (LSPs) Ping Parameters" registry under "TLVs"
([IANA-MPLS-LSP-PING]).
Sub-Type Sub-TLV Name Reference
-------- ------------ ---------
33 Entropy label FEC this document
11.2. DS Flags
IANA has assigned new bit numbers from the "DS Flags" subregistry
from the "TLVs" section of the "Multi-Protocol Label Switching (MPLS)
Label Switched Paths (LSPs) Ping Parameters" registry
([IANA-MPLS-LSP-PING]).
Note: The "DS Flags" subregistry was created by [RFC7537].
Bit number Name Reference
---------- ---------------------------------------- ---------
5 E: ELI/EL push indicator this document
4 L: Label-based load balance indicator this document
11.3. Multipath Type
IANA has assigned a new value from the "Multipath Type" subregistry
from the "TLVs" section of the "Multi-Protocol Label Switching (MPLS)
Label Switched Paths (LSPs) Ping Parameters" registry
([IANA-MPLS-LSP-PING]).
Note: The "Multipath Type" subregistry was created by [RFC7537].
Value Meaning Reference
---------- ---------------------------------------- ---------
10 IP and label set this document
12. References
12.1. 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>.
[RFC6424] Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
Performing Label Switched Path Ping (LSP Ping) over MPLS
Tunnels", RFC 6424, DOI 10.17487/RFC6424, November 2011,
<http://www.rfc-editor.org/info/rfc6424>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<http://www.rfc-editor.org/info/rfc6790>.
[RFC7537] Decraene, B., Akiya, N., Pignataro, C., Andersson, L., and
S. Aldrin, "IANA Registries for LSP Ping Code Points",
RFC 7537, DOI 10.17487/RFC7537, May 2015,
<http://www.rfc-editor.org/info/rfc7537>.
12.2. Informative References
[IANA-MPLS-LSP-PING]
IANA, "Multi-Protocol Label Switching (MPLS) Label
Switched Paths (LSPs) Ping Parameters",
<http://www.iana.org/assignments/mpls-lsp-ping-parameters>.
[RFC6391] Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
Regan, J., and S. Amante, "Flow-Aware Transport of
Pseudowires over an MPLS Packet Switched Network",
RFC 6391, DOI 10.17487/RFC6391, November 2011,
<http://www.rfc-editor.org/info/rfc6391>.
[RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
and C. Pignataro, "MPLS Forwarding Compliance and
Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
August 2014, <http://www.rfc-editor.org/info/rfc7325>.
Acknowledgements
The authors would like to thank Loa Andersson, Curtis Villamizar,
Daniel King, Sriganesh Kini, Victor Ji, Acee Lindem, Deborah
Brungard, Shawn M Emery, Scott O. Bradner, and Peter Yee for
performing thorough reviews and providing very valuable comments.
Carlos Pignataro would like to acknowledge his lifetime friend Martin
Rigueiro, with deep gratitude and esteem, for sharing his contagious
passion for engineering and sciences, and for selflessly teaching so
many lessons.
Contributors
Nagendra Kumar
Cisco Systems, Inc.
Email: naikumar@cisco.com
Authors' Addresses
Nobo Akiya
Big Switch Networks
Email: nobo.akiya.dev@gmail.com
George Swallow
Cisco Systems, Inc.
Email: swallow@cisco.com
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Andrew G. Malis
Huawei Technologies
Email: agmalis@gmail.com
Sam Aldrin
Google
Email: aldrin.ietf@gmail.com
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