PMS/Head-end based MPLS Ping and Traceroute in Inter-domain SR Networks
draft-ietf-mpls-spring-inter-domain-oam-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
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|---|---|---|---|
| Authors | Shraddha Hegde , Kapil Arora , Mukul Srivastava , Samson Ninan , Nagendra Kumar Nainar | ||
| Last updated | 2022-01-02 (Latest revision 2021-12-13) | ||
| Replaces | draft-ninan-mpls-spring-inter-domain-oam | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Send notices to | loa@pi.nu |
draft-ietf-mpls-spring-inter-domain-oam-02
Routing area S. Hegde
Internet-Draft K. Arora
Intended status: Standards Track M. Srivastava
Expires: 16 June 2022 Juniper Networks Inc.
S. Ninan
Ciena
N. Kumar
Cisco Systems, Inc.
13 December 2021
PMS/Head-end based MPLS Ping and Traceroute in Inter-domain SR Networks
draft-ietf-mpls-spring-inter-domain-oam-02
Abstract
Segment Routing (SR) architecture leverages source routing and
tunneling paradigms and can be directly applied to the use of a
Multiprotocol Label Switching (MPLS) data plane. A network may
consist of multiple IGP domains or multiple ASes under the control of
same organization. It is useful to have the Label switched Path
(LSP) Ping and traceroute procedures when an SR end-to-end path spans
across multiple ASes or domains. This document describes mechanisms
to facilitae LSP ping and traceroute in inter-AS/inter-domain SR-MPLS
networks in an efficient manner with simple Operations,
Administration, and Maintenance (OAM) protocol extension which uses
dataplane forwarding alone for sending echo reply.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 https://datatracker.ietf.org/drafts/current/.
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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 16 June 2022.
Copyright Notice
Copyright (c) 2021 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 (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definition of Domain . . . . . . . . . . . . . . . . . . 5
2. Inter domain networks with multiple IGPs . . . . . . . . . . 5
3. Reply Path TLV . . . . . . . . . . . . . . . . . . . . . . . 5
4. Segment sub-TLV . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Type A: SID only, in the form of MPLS Label . . . . . . . 7
4.2. Type C: IPv4 Node Address with optional SID for
SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Type D: IPv6 Node Address with optional SID for SR
MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Segment Flags . . . . . . . . . . . . . . . . . . . . . . 10
5. SRv6 Dataplane . . . . . . . . . . . . . . . . . . . . . . . 11
6. Detailed Procedures . . . . . . . . . . . . . . . . . . . . . 11
6.1. Sending an echo request . . . . . . . . . . . . . . . . . 11
6.2. Receiving an echo request . . . . . . . . . . . . . . . . 11
6.3. Sending an echo reply . . . . . . . . . . . . . . . . . . 12
6.4. Receiving an echo reply . . . . . . . . . . . . . . . . . 12
7. Detailed Example . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Procedures for Segment Routing LSP ping . . . . . . . . . 13
7.2. Procedures for Segment Routing LSP Traceroute . . . . . . 14
8. Building Reply Path TLV dynamically . . . . . . . . . . . . . 16
8.1. The procedures to build the return path . . . . . . . . . 16
8.2. Details with example . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
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11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
+----------------+
| Controller/PMS |
+----------------+
|---AS1-----| |------AS2------| |----AS3---|
ASBR2----ASBR3 ASBR5------ASBR7
/ \ / \
/ \ / \
PE1----P1---P2 P3---P4---PE4 P5---P6--PE5
\ / \ /
\ / \ /
ASBR1----ASBR4 ASBR6------ASBR8
Figure 1: Inter-AS Segment Routing topology
Many network deployments have built their networks consisting of
multiple Autonomous Systems either for ease of operations or as a
result of network mergers and acquisitions. Segment Routing can be
deployed in such scenarios to provide end to end paths, traversing
multiple Autonomous systems(AS). These paths consist of Segment
Identifiers(SID) of different type as per [RFC8402].
[RFC8660] specifies the forwarding plane behaviour to allow Segment
Routing to operate on top of MPLS data plane. [RFC9087] describes
BGP peering SIDs, which will help in steering packet from one
Autonomous system to another. Using above SR capabilities, paths
which span across multiple Autonomous systems can be created.
For example Figure 1 describes an inter-AS network scenario
consisting of ASes AS1 and AS2. Both AS1 and AS2 are Segment Routing
enabled and the EPE links have EPE labels configured and advertised
via [I-D.ietf-idr-bgpls-segment-routing-epe]. Controller or head-end
can build end-to-end Traffic-Engineered path consisting of Node-SIDs,
Adjacency-SIDs and EPE-SIDs. It is advantageous for operations to be
able to perform LSP ping and traceroute procedures on these inter-AS
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SR-MPLS paths. LSP ping/traceroute procedures use IP connectivity
for echo reply to reach the head-end. In inter-AS networks, IP
connectivity may not be there from each router in the path.For
example in Figure 1 P3 and P4 may not have IP connectivity for PE1.
[RFC8403] describes mechanisms to carry out the MPLS ping/traceroute
from a Path Monitoring System (PMS). It is possible to build GRE
tunnels or static routes to each router in the network to get IP
connectivity for the reverse path. This mechanism is operationally
very heavy and requires PMS to be capable of building huge number of
GRE tunnels, which may not be feasible.
It is not possible to carry out LSP ping and Traceroute functionality
on these paths to verify basic connectivity and fault isolation using
existing LSP ping and Traceroute mechanism([RFC8287] and [RFC8029]).
This is because, there exists no IP connectivity to source address of
ping packet, which is in a different AS, from the destination of
Ping/Traceroute.
[RFC7743] describes a Echo-relay based solution based on advertising
a new Relay Node Address Stack TLV containing stack of Echo-relay IP
addresses. These mechansims can be applied to segment routing
networks as well. [RFC7743] mechanism requires the return ping
packet to be processed in slow path or as a bump-in-the-wire on every
relay node. The motivation of the current document is to provide an
alternate mechanism for ping/traceroute in inter-domain segment
routing networks.
This document describes a new mechanism which is efficient and simple
and can be easily deployed in SR-MPLS networks. This mechanism uses
MPLS path and no changes required in the forwarding path. Any MPLS
capable node will be able to forward the echo-reply packet in fast
path. The current draft describes a mechanism that uses Reply path
TLV [RFC7110] to convey the reverse path. Three new sub-TLVs for
Reply path TLV are defined, that faciliate encoding segment routing
label stack. The TLV can either be derived by a smart application or
controller which has a full topology view. This document also
proposes mechanisms to derive the return path dynamically during
traceroute procedures.
The current document is focused on the inter-domain use case.
However, the protocol extensions described in this document may be
applied to indicate the return path for other use cases as well.
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1.1. Definition of Domain
The term domain used in this document implies an IGP domain where
every node is visible to every other node for the purposes of
shortest path computation. The domain implies an IGP area or level.
An Autonomous System (AS) consists of one or more IGP domains.The
procedures described in this document are applicable to paths built
across multiple domains which includes inter-area as well as inter-AS
paths. This document is applicable to SR-MPLS networks where all
nodes in each of the domains are SR capable. It is also applicable
to SR-MPLS networks where SR acts an an overlay having SR incapable
underlay nodes. In such networks, the traceroute procedure is
executed only on the overlay SR nodes.
2. Inter domain networks with multiple IGPs
|-Domain 1|-------Domain 2-----|--Domain 3-|
PE1------ABR1--------P--------ABR2------PE4
\ / \ /\ /
-------- ----------------- -------
BGP-LU BGP-LU BGP-LU
Figure 2: Inter-domain networks with multiple IGPs
When the network consists of large number of nodes, the nodes are
seggregated into multiple IGP domains. The connectivity to the
remote PEs can be achieved using BGP-Labeled Unicast (BGP-LU)
[RFC8277] or by stacking the labels for each domain as described in
[RFC8604]. It is useful to support MPLS ping and traceroute
mechanisms for these networks. The procedures described in this
document for constructing Reply path TLV and its use in echo reply is
equally applicable to networks consisting of multiple IGP domains
that use BGP-LU or label stacking.
3. Reply Path TLV
Segment Routing networks statically assign the labels to nodes and
PMS/Head-end may know the entire database. The reverse path can be
built from PMS/Head-end by stacking segments for the reverse path.
Reply path TLV as defined in [RFC7110] is used to carry the return
path. While using the procedures described in this document, the
reply mode is set to 5 (Reply via Specified Path), and Reply Path TLV
is included in the echo request message as described in [RFC7110].
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The Reply Path TLV is constructed as per Section 4.2 of RFC 7110.This
document define three new sub-TLVs to encode the Segment Routing
path.
The type of segment that the head-end chooses to send in the Reply
Path TLV is governed by local policy. Implementations may provide
CLI input parameters in Labels, IPv4 addresses or IPv6 addresses or a
combination of these which gets encoded in the return path TLV.
Implementations may also provide mechansims to acquire the database
of remote domains and compute the return path based on the acquired
database. For traceroute purposes, the return path will have to
consider the reply being sent from every node along the path. The
return path changes when the traceroute progresses and crosses each
domain. One of the ways this can be implemented on headend is to
acquire the entire database (of all domains) and build return path
for every node along the SR-MPLS path based on the knowledge of the
database. Another mechansim is to use dynamically computed return
path as described in Section 8
Some networks may consist of pure IPV4 domains and pure IPv6 domains.
Handling end-to-end MPLS OAM for such networks is out of scope for
this document. It is recommended to use dual stack in such cases and
use end-to-end IPv6 addresses for MPLS ping and trace route
procedures.
4. Segment sub-TLV
[I-D.ietf-spring-segment-routing-policy] defines various types of
segments. The types of segments applicable to this document have
been defined in this section for the use of MPLS OAM. The motivation
has been to keep the definitions same as in
[I-D.ietf-spring-segment-routing-policy] with minimal modifications
if it is absolutely needed. One or more segment sub-TLV can be
included in the Reply Path TLV. The segment sub-TLVs included in a
Reply Path TLV MAY be of different types.
Below types of segment sub-TLVs are applicable for the Reverse Path
Segment List TLV.
Type A: SID only, in the form of MPLS Label
Type C: IPv4 Node Address with optional SID
Type D: IPv6 Node Address with optional SID for SR MPLS
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4.1. Type A: SID only, in the form of MPLS Label
The Type-A Segment Sub-TLV encodes a single SID in the form of an
MPLS label. The format 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Type 1 Segment sub-TLV
where:
Type: TBD1(to be assigned by IANA from the registry "Sub-TLVs for TLV
Types 1, 16, and 21").
Length is 8.
Flags: 1 octet of flags as defined in Section 4.4.
RESERVED: 3 octets of reserved bits. SHOULD be unset on transmission
and MUST be ignored on receipt.
Label: 20 bits of label value.
TC: 3 bits of traffic class
S: 1 bit Reserved
TTL: 1 octet of TTL.
The following applies to the Type-1 Segment sub-TLV:
The S bit SHOULD be zero upon transmission, and MUST be ignored upon
reception.
If the originator wants the receiver to choose the TC value, it sets
the TC field to zero.
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If the originator wants the receiver to choose the TTL value, it sets
the TTL field to 255.
If the originator wants to recommend a value for these fields, it
puts those values in the TC and/or TTL fields.
The receiver MAY override the originator's values for these fields.
This would be determined by local policy at the receiver. One
possible policy would be to override the fields only if the fields
have the default values specified above.
4.2. Type C: IPv4 Node Address with optional SID for SR-MPLS
The Type-C Segment Sub-TLV encodes an IPv4 node address, SR Algorithm
and an optional SID in the form of an MPLS label. The format 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | RESERVED (MBZ) | SR Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Node Address (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (optional, 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Type 3 Segment sub-TLV
where:
Type: TBD3(to be assigned by IANA from the registry "Sub-TLVs for TLV
Types 1, 16, and 21").
Length is 8 or 12.
Flags: 1 octet of flags as defined in Section 4.4.
SR Algorithm: 1 octet specifying SR Algorithm as described in section
3.1.1 in [RFC8402], when A-Flag as defined in Section 4.4is present.
SR Algorithm is used by the receiver to derive the Label. When
A-flag is unset, this field has no meaning and thus MUST be set to
zero on transmission and ignored on receipt.
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RESERVED: 2 octets of reserved bits.MUST be set to zero when sending;
MUST be ignored on receipt.
IPv4 Node Address: a 4 octet IPv4 address representing a node.
SID: optional :4 octet field containing label, TC, S and TTL as
defined in Section 4.1
The following applies to the Type-3 Segment sub-TLV:
The IPv4 Node Address MUST be present.
The SID is optional and specifies a 4 octet MPLS SID containing
label, TC, S and TTL as defined in Section 4.1.
If length is 8, then only the IPv4 Node Address is present.
If length is 12, then the IPv4 Node Address and the MPLS SID are
present.When the MPLS SID field is present, it MUST be used for
constructing the Reply Path TLV.
4.3. Type D: IPv6 Node Address with optional SID for SR MPLS
The Type-D Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
and an optional SID in the form of an MPLS label. The format 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | RESERVED(MBZ) | SR Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// IPv6 Node Address (16 octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (optional, 4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Type 4 Segment sub-TLV
where:
Type: TBD4(to be assigned by IANA from the registry "Sub-TLVs for TLV
Types 1, 16, and 21").
Length is 20 or 24.
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Flags: 1 octet of flags as defined in Section 4.4.
SR Algorithm: 1 octet specifying SR Algorithm as described in section
3.1.1 in [RFC8402], when A-Flag as defined in Section 4.4 is present.
SR Algorithm is used by the receiver to derive the label.W hen A-flag
is unset, this field has no meaning and thus MUST be set to zero on
transmission and ignored on receipt.
RESERVED: 2 octets of reserved bits.MUST be set to zero when sending;
MUST be ignored on receipt..
IPv6 Node Address: a 16 octet IPv6 address representing a node.
SID: optional :4 octet field containing label, TC, S and TTL as
defined in Section 4.1
The following applies to the Type-4 Segment sub-TLV:
The IPv6 Node Address MUST be present.
The SID is optional and specifies a 4 octet MPLS SID containing
label, TC, S and TTL as defined in Section 4.1 .
If length is 20, then only the IPv6 Node Address is present.
If length is 24, then the IPv6 Node Address and the MPLS SID are
present. When the MPLS SID field is present, it MUST be used for
constructing the Reply Path TLV.
4.4. Segment Flags
The Segment Types described above contain following flags in the
"Flags" field (codes to be assigned by IANA from the registry
"Segment sub-TLV Flags" )
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |A| |
+-+-+-+-+-+-+-+-+
Figure 6: Flags
where:
A-Flag: This flag indicates the presence of SR Algorithm id in the
"SR Algorithm" field applicable to various Segment Types.
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Unused bits in the Flag octet SHOULD be set to zero upon transmission
and MUST be ignored upon receipt.
The following applies to the Segment Flags:
A-Flag is applicable to Segment Types 3, 4. If A-Flag appears with
any other Segment Type, it MUST be ignored.
5. SRv6 Dataplane
SRv6 dataplane is not in the scope of this document and will be
addressed in a separate document.
6. Detailed Procedures
6.1. Sending an echo request
In the inter-AS scenario when there is no reverse path connectivity,
the procedures described in this document should be used. LSP ping
initiator MUST set the Reply Mode of the echo request to "Reply via
Specified Path", and a Reply Path TLV MUST be carried in the echo
request message correspondingly. The Reply Path TLV must contain the
Segment Routing Path in the reverse direction encoded as an ordered
list of segments. The first Segment MUST correspond to the top
Segment in MPLS header that the responder MUST use while sending the
echo reply.
6.2. Receiving an echo request
As described in [RFC7110], when Reply mode is set to 5 (Reply via
Specified Path),The echo request MUST contain the Reply path TLV.
Absence of Reply path TLV is treated as malformed echo request. when
an echo request is received, if the egress LSR does not know the
Reply Mode 5 defined in [RFC7110], an echo reply with the return code
set to "Malformed echo request received" and the Subcode set to zero
will be sent back to the ingress LSR according to the rules of
[RFC8029]. When a Reply Path TLV is received, and the responder that
supports processing it, it MUST use the segments in Reply Path TLV to
build the echo reply.The responder MUST follow the normal FEC
validation procedures as described in [RFC8029] and [RFC8287] and
this document does not suggest any change to those procedures. When
the echo reply has to be sent out the Reply Path TLV is used to
construct the MPLS packet to send out.
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6.3. Sending an echo reply
The echo reply message is sent as MPLS packet with a MPLS label
stack. The echo reply message MUST be constructed as described in
the [RFC8029]. An MPLS packet is constructed with echo reply in the
payload. The top label MUST be constructed from the first Segment
from the Reply Path TLV. The remaining labels MUST follow the order
from the Reply Path TLV. The responder MAY check the reachability of
the top label in its own Label Forwarding Information Base (LFIB)
before sending the echo reply. In certain scenarios the head-end may
choose to send Type 3/Type 4 segments consisting of IPV4 address or
IPv6 address. Optionally a SID may also be assiciated with Type 3/
Type4 segment. In such cases the node sending the echo reply MUST
derive the MPLS labels based on Node-SIDs associated with the IPv4
/IPv6 addresses or from the optional MPLS SIDs in the type 3/ type 4
segments and encode the echo reply with MPLS labels.
The reply path return code MUST be set as described in section 7.4 of
[RFC7110]. The Reply Path TLV MUST be included in echo reply
indicating the specified return path that the echo reply message is
required to follow as described in section 5.3 of [RFC7110].
When the node is configured to dynamically create return path for
next echo request, the procedures described in Section 8 MUST be
used. The reply path return code MUST be set to TBA1 and same Reply
Path TLV or a new Reply Path TLV MUST be included in the echo reply.
6.4. Receiving an echo reply
The rules and process defined in Section 4.6 of [RFC8029] and section
5.4 of [RFC7110] apply here. In addition, if the Reply path return
code is "Use Reply Path TLV in echo reply for next echo request", the
Reply Path TLV from the echo Reply MUST be sent in the next echo
request with TTL incremented by 1.
7. Detailed Example
Example topologies given in Figure 1 and Figure 2 will be used in
below sections to explain LSP Ping and Traceroute procedures. The
PMS/Head-end has complete view of topology. PE1, P1, P2, ASBR1 and
ASBR2 are in AS1. Similarly ASBR3, ASBR4, P3, P4 and PE4 are in AS2.
AS1 and AS2 have Segment Routing enabled. IGPs like OSPF/ISIS are
used to flood SIDs in each Autonomous System. The ASBR1, ASBR2,
ASBR3, ASBR4 advertise BGP EPE SIDs for the inter-AS links. Topology
of AS1 and AS2 are advertised via BGP-Link State (BGP-LS) to the
controller/PMS or Head-end node. The EPE-SIDs are also advertised
via BGP-LS as described in [I-D.ietf-idr-bgpls-segment-routing-epe]
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The description in the document uses below notations for Segment
Identifiers(SIDs).
Node SIDs : N-PE1, N-P1, N-ASBR1 N-ABR1, N-ABR2etc.
Adjacency SIDs : Adj-PE1-P1, Adj-P1-P2 etc.
EPE SIDS : EPE-ASBR2-ASBR3, EPE-ASBR1-ASBR4, EPE-ASBR3-ASBR2 etc.
Let us consider a traffic engineered path built from PE1 to PE4 with
Segment List stack as below. N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4
for following procedures. This stack may be programmed by
controller/PMS or Head-end router PE1 may have imported the whole
topology information from BGP-LS and computed the inter-AS path.
7.1. Procedures for Segment Routing LSP ping
To perform LSP ping procedure on an SR-MPLS-Path from PE1 to PE4
consisting of label stacks [N-P1,N-ASBR1,EPE-ASBR1-ASBR4, N-PE4], The
remote end(PE4) needs IP connectivity to head end(PE1) for the
Segment Routing ping to succeed, because echo reply needs to travel
back to PE1 from PE4. But in typical deployment scenario there will
be no IP route from PE4 to PE1 as they belong to different ASes.
PE1 adds return Path from PE4 to PE1 in the MPLS echo request using
multiple Segments in "Reply Path TLV" as defined above. An example
return path TLV for PE1 to PE4 for LSP ping is [N-ASBR4, EPE-
ASBR4-ASBR1, N-PE1]. An implementation may also build a return Path
consisting of labels to reach its own AS. Once the label stack is
popped-off the echo reply message will be exposed. The further
packet forwarding will be based on IP lookup. An example return Path
for this case could be [N-ASBR4, EPE-ASBR4-ASBR1].
On receiving MPLS echo request PE4 first validates FEC in the echo
request. PE4 then builds label stack to send the response from PE4
to PE1 by copying the labels from "Reply Path TLV". PE4 builds the
echo reply packet with the MPLS label stack constructed and imposes
MPLS headers on top of echo reply packet and sends out the packet
towards PE1. This Segment List stack can successfully steer reply
back to Head-end node(PE1).
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7.2. Procedures for Segment Routing LSP Traceroute
Traceroute procedure involves visiting every node on the path and
echo reply sent from every node. In this section, we describe the
traceroute mechanims when the headend/PMS has complete visibility of
the database. Headend/PMS computes the return path from each node in
the entire SR-MPLS path that is being tracerouted. The return path
computation is implementation dependant. As the headend/PMS
completely controls the return path, it can use proprietary
computations to build the return path.
One of the ways the return path can be built, is to use the principle
of building label stacks by adding each domain border node's Node SID
on the return path label stack as the traceroute progresses. For
inter-AS networks, in addition to border node's Node-SID, EPE-SID in
the reverse direction also need to be added to the label stack.
The Inter-domain/inter-as traceroute procedure uses the TTL expiry
mechansim as specified in [RFC8029] and [RFC8287]. Every echo
request packet Headend/PMS MUST include the appropriate return path
in the Reply Path TLV. The node that receives the echo request MUST
follow procedures described in section Section 6.1 and section
Section 6.2 to send out echo reply.
For Example:
Let us consider a topology from Figure 1. Let us consider a SR-MPLS
path [N-P1,N-ASBR1,EPE-ASBR1-ASBR4, N-PE4]. The traceroute is being
executed for this inter-AS path for destination PE4. PE1 sends first
echo request with TTL set to 1 and includes return path TLV
consisting of Type 1 Segment containing label derived from its own SR
Global Block (SRGB). Note that the type of segment used in
constructing the return Path is local policy. If the entire network
has same SRGB configured, Type 1 segments can be used.The TTL expires
on P1 and the P1 sends echo reply using the return path. Note that
implementations may choose to exclude return path TLV until
traceroute reaches the first domain border as the return IP path to
PE1 is expected to be available inside the first domain.
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TTL is set to 2 and the next echo request is sent out. Until the
traceroute procedure reaches the domain border node ASBR1, same
return path TLV consisting of single Label (PE1's node Label)is used.
When echo request reaches ASBR1, and echo reply is received, the next
echo request needs to include additional label as ASBR1 is a border
node. The return path TLV is built based on the forward path. As
the forward path consists of EPE-ASBR1-ASBR4, an EPE-SID in the
reverse direction is included in the return path TLV. The return
path now consists of two labels [N-PE1, EPE-ASBR4-ASBR1]. The echo
reply from ASBR4 will use this return path to send the reply.
The next echo request after visiting the border node ASBR4 will
update the return path with Node-SID label of ASBR4. The return path
beyond ASBR4 will be [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4]. This same
return path is used until the traceroute procedure reaches next set
of border nodes. When there are multiple ASes the traceroute
procedure will continue by adding a set of Node labels and EPE labels
as the border nodes are visited.
Note that the above return path building procedure requires the
database of all the domains to be available at the headend/PMS.
The above description assumed the same SRGB is configured on all
nodes along the path. The SRGB may differ from one node to another
node and the SR architecture [RFC8402] allows the nodes to use
different SRGB. In such scenarios PE1 sends Type 3 (or Type 4 in
case of IPv6 networks) segment with Node address of PE1 and with
optional MPLS SID associated with the Node address. The receiving
node derives the label for the return path based on its own SRGB.
When the traceroute procedure crosses the border ASBR1, headend PE1
should send type 1 segment for N-PE1 based on the label derived from
ASBR1's SRGB. This is required because in AS2, ASBR4, P3, P4 etc may
not have the topology information to derive SRGB for PE1. After the
traceroute procedure reaches ASBR4 the return path will be
[N-PE1(type1 with label based on ASBR1's SRGB), EPE-ASBR4-ASBR1,
N-ASBR4 (Type 3)].
In order to extend the example to multiple ASes consisting of 3 or
more ASes, let us consider a traceroute from PE1 to PE5 in Figure 1.
In this example, the PE1 to PE5 path has to cross 3 domains AS1, AS2
and AS3. Let us consider a path from PE1 to PE5 that goes through
[PE1, ASBR1, ASBR4, ASBR6, ASBR8,PE5]. When the traceroute procedure
is visiting the nodes in AS1, the Reply path TLV sent from headend
consists of [N-PE1]. When the traceroute procedure reaches the
ASBR4, the return Path consists of [N-PE1, EPE-ASBR4-ASBR1]. While
visiting nodes in AS2, the traceroute procedure consists of Reply
Path TLV [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4]. similarly, while visiting
the ASBR8 Reply Path TLV adds the EPE SID from ASBR8 to ASBR6. While
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visiting nodes in AS3 Node-SId of ASBR8 would also be added which
makes the return Path [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4, EPE-
ASBR8-ASBR6, N-ASBR8]
Let us consider another example from topology Figure 2. This
topology consists of multi-domain IGP with common border node between
the domains. This could be achieved with multi-area or multi-level
IGP or multiple instances of IGP deployed on same node. The return
path computation for this topology is similar to the multi-AS
computation except that the return path consists of single border
node label. When traceroute procedure is visiting node P, the return
path consists of [N-PE1, N-ABR1].
8. Building Reply Path TLV dynamically
In some cases, the head-end may not have complete visibility of
Inter-AS/Inter-domain topology. In such cases, it can rely on
downstream routers to build the reverse path for MPLS traceroute
procedures. For this purpose, Reply Path TLV in the echo reply
corresponds to the return path to be used in next echo request.
Value Meaning
------ ----------------------
TBA1 Use Reply Path TLV in echo reply for next echo request.
Figure 7: Reply path return Code
8.1. The procedures to build the return path
In order to dynamically build the return Path for traceroute
procedures, the domain border nodes along the path being tracerouted
MUST support the procedures described in this section. Local policy
on the domain border nodes SHOULD determine whether the domain border
node participates in building return path dynamically during
traceroute.
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Headend/PMS node MAY include its own node label while initiating
traceroute procedure. When an ABR receives the echo request, if the
local policy implies building dynamic return path, ABR MUST include
its own Node label. If there is a Reply Path TLV included in the
received echo request message, the ABR's node label is added before
the existing segments. The type of segment added is based on local
policy. In cases when SRGB is not uniform across the network, it is
RECOMMENDED to add type 3 or type 4 segment. If the existing segment
in the Reply Path TLV is a type 3/type 4 segment, that segment MUST
be converted to Type 1 segment based on ABR's own SRGB.This is
because downstream nodes will not know what SRGB to use to translate
the IP address to a label. As the ABR added its own Node label, it
is guaranteed that this ABR will be in the return path and will be
forwarding the traffic based on next label after its own label.
When an ASBR receives an echo request from another AS, and ASBR is
configured to build the return path dynamically, ASBR MUST build a
Reply Path TLV and include it in the echo reply. The Reply Path TLV
MUST consist of its own node label and an EPE-SID to the AS from
where the traceroute message was received. A Reply path return code
of TBA1 MUST be set in the echo reply to indicate that next echo
request should use the return Path from the Reply Path TLV in the
echo reply. ASBR MUST locally decide the outgoing interface for the
echo reply packet. Generally, remote ASBR will choose interface on
which the incoming OAM packet was receieved to send the echo reply
out. Reply Path TLV is built by adding two segment sub TLVs. The
top segment sub TLV consists of the ASBR's Node SID and second
segment consists of the EPE SID in the reverse direction to reach the
AS from which the OAM packet was received.The type of segment chosen
to build Reply Path TLV is a local policy. It is RECOMMENDED to use
type 3/type4 segment for the top segment when the SRGB is not
gurateed to be uniform in the domain.
Irrespective of which type of segment is included in the Reply Path
TLV, the responder of echo request MUST always translate the Reply
Path TLV to a label stack and build MPLS header for the the echo
reply packet. This procedure can be applied to an end-to-end path
consisting of multiple ASes. Each ASBR that receives echo request
from another AS adds its Node-SID and EPE-SID on top of existing
segments in the Reply Path TLV.
An ASBR that receives the echo request from a neighbor belonging to
same AS, MUST look at the Reply Path TLV received in the echo
request. If the Reply Path TLV consists of a Type 3/Type 4 segment,
it MUST convert the Type 3/4 segment to Type 1 segment by deriving
label from its own SRGB. The ASBR MUST set the reply path return
code to TBA1 and send the newly constructed Reply Path TLV in the
echo reply.
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Internal nodes or non domain border nodes MAY not set the Reply Path
TLV return code to TBA1 in the echo reply message as there is no
change in the return Path. In these cases, the headend node/PMS that
initiates the traceroute procedure MUST continue to send previously
sent Reply Path TLV in the echo request message in every next echo
request.
Note that an ASBR's local policy may prohibit it from participating
in the dynamic traceroute procedures. If such an ASBR is encountered
in the forward path, dynamic return path building procedures will
fail. In such cases, ASBR that supports this document MUST set the
return code TBA2 to indicate local policies do not allow the dynamic
return path building.
Value Meaning
------ ---------------------------------------------------
TBA2 Local policy does not allow dynamic return Path building.
Figure 8: Local policy return Code
8.2. Details with example
Let us consider a topology from Figure 1. Let us consider a SR
policy path built from PE1 to PE4 with a label stack as below. N-P1,
N-ASBR1, EPE-ASBR1-ASBR4, N-PE4. PE1 begins traceroute with TTL set
to 1 and includes [N-PE1] in the Reply Path TLV. The traceroute
packet TTL expires on P1 and P1 processes the traceroute as per the
procedures described in [RFC8029] and [RFC8287]. P1 sends echo reply
with the same return Path TLV with reply path return code set to 6.
The return code of the echo reply itself is set to the return code as
per [RFC8029] and [RFC8287]. This traceroute doesn't need any
changes to the Reply Path TLV till it leaves AS1. The same Reply
Path TLV that is received may be included in the echo reply by P1 and
P2 or no Reply Path TLV included so that headend continues to use
same return path in echo request that it used to send previous echo
request.
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When ASBR1 receives the echo request, in case it recieved type3/type
4 segment in the Reply Path TLV in the echo request, it converts that
type 3/4 segment to Type 1 based on its own SRGB. When ASBR4
receives the echo request, it should form this Reply Path TLV using
its own Node SID(N-ASBR4) and EPE SID (EPE-ASRB4-ASBR1) labels and
set the reply path return code to TBA1. Then PE1 should use this
Reply Path TLV in subsequent echo requests. In this example, when
the subsequent echo request reaches P3, it should use this Reply Path
TLV for sending the echo reply. The same Reply Path TLV is
sufficient for any router in AS2 to send the reply. Because the
first label(N-ASBR4) can direct echo reply to ASBR4 and second one
(EPE-ASBR4-ASBR1) to direct echo reply to AS1. Once echo reply
reaches AS1, normal IP forwarding or the N-PE1 helps it to reach PE1.
The example described in above paragraphs can be extended to multiple
ASes by following the same procedure of each ASBR adding Node-SID and
EPE-SID on receieving echo request from neighboring AS.
Let us consider a topology from Figure 2. It consists of multiple
IGP domains with multiple area/levels or separate IGP instances.
There is a single border node that seperates the two domains. In
this case, PE1 sends traceroute packet with TTL set to 1 and includes
N-PE1 in the return path TLV. ABR1 receives the echo request and
while sending echo reply adds its own node Label to the Reply Path
TLV and sets the Reply path return code to TBA1. The Reply path TLV
in the echo reply from ABR1 consists of [N-PE1, N-ABR1]. Next echo
request with TTL 2 reaches P node. It is an internal node so it does
not change the return Path. echo request with TTL 3 reaches ABR2 and
it adds its own Node label so the return path TLV sent in echo reply
will be [N-PE1, N-ABR1, N-ABR2]. echo request with TTL 4 reaches PE4
and it sends echo reply return code as Egress. PE4 does not include
any Reply Path TLV in echo reply. The above example assumes uniform
SRGB throughout the domain. In case of different SRGBs, the top
segment will be a type 3/4 segment and all other segments will be
type 1. Each border node converts the type 3/type 4 segment to type
1 before adding its own segment to the Reply Path TLV.
9. Security Considerations
The procedures described in this document enable LSP ping and
traceroute to be executed across multiple domains or multiple ASes
that belong to same adminstration or closely co-operating
administration. It is assumed that sharing domain internal
information across such domains does not pose security risk. However
procedures described in this document may be used by an attacker to
extract the domain internal information. An operator MUST deploy
appropriate filter policies as described in [RFC8029] to restrict the
LSP ping/traceroute packets based on origin. It is also suggested
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that an operator SHOULD deploy security mechanisms such as MACSEC on
inter-domain links or security vulnerable links to prevent spoofing
attacks.
10. IANA Considerations
Sub-TLVs for TLV Types 1, 16, and 21
SID only in the form of MPLS label : TBD (Range 32768-65535)
IPv4 Node Address with optional SID for SR-MPLS : TBD (Range
32768-65535)
IPv6 Node Address with optional SID for SR-MPLS : TBD (Range
32768-65535)
Reply Path Return Codes Registry
TBA1:Use Reply Path TLV in echo reply for next echo request.
TBA2:Local policy does not allow dynamic return Path building.
11. Contributors
1.Carlos Pignataro
Cisco Systems, Inc.
cpignata@cisco.com
2. Zafar Ali
Cisco Systems, Inc.
zali@cisco.com
12. Acknowledgments
Thanks to Bruno Decreane for suggesting use of generic Segment sub-
TLV. Thanks to Adrian Farrel, Huub van Helvoort, Dhruv Dhody,
Dongjie, for careful review and comments. Thanks to Mach Chen for
suggesting to use Reply Path TLV. Thanks to Gregory Mirsky for
detailed review which helped improve the readability of the document
to a great extent.
13. References
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13.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc7110>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8287] Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
N., Kini, S., and M. Chen, "Label Switched Path (LSP)
Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
<https://www.rfc-editor.org/info/rfc8287>.
[RFC9087] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Aries, E.,
and D. Afanasiev, "Segment Routing Centralized BGP Egress
Peer Engineering", RFC 9087, DOI 10.17487/RFC9087, August
2021, <https://www.rfc-editor.org/info/rfc9087>.
13.2. Informative References
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing BGP Egress Peer
Engineering", Work in Progress, Internet-Draft, draft-
ietf-idr-bgpls-segment-routing-epe-19, 16 May 2019,
<https://www.ietf.org/archive/id/draft-ietf-idr-bgpls-
segment-routing-epe-19.txt>.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
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Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-14, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-spring-
segment-routing-policy-14.txt>.
[RFC7743] Luo, J., Ed., Jin, L., Ed., Nadeau, T., Ed., and G.
Swallow, Ed., "Relayed Echo Reply Mechanism for Label
Switched Path (LSP) Ping", RFC 7743, DOI 10.17487/RFC7743,
January 2016, <https://www.rfc-editor.org/info/rfc7743>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8403] Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
2018, <https://www.rfc-editor.org/info/rfc8403>.
[RFC8604] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed.,
Henderickx, W., and D. Cooper, "Interconnecting Millions
of Endpoints with Segment Routing", RFC 8604,
DOI 10.17487/RFC8604, June 2019,
<https://www.rfc-editor.org/info/rfc8604>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
Authors' Addresses
Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore 560103
KA
India
Email: shraddha@juniper.net
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Kapil Arora
Juniper Networks Inc.
Email: kapilaro@juniper.net
Mukul Srivastava
Juniper Networks Inc.
Email: msri@juniper.net
Samson Ninan
Ciena
Email: sninan@ciena.com
Nagendra Kumar
Cisco Systems, Inc.
Email: naikumar@cisco.com
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