RFC 8611
Label Switched Path (LSP) Ping and Traceroute Multipath Support for Link Aggregation Group (LAG) Interfaces
5. Mechanism to Validate L2 ECMP Traversal
Section 4 defines the responder LSR procedures to construct a DDMAP for a downstream LAG. The Remote Interface Index Sub-TLV that describes the incoming LAG member links of the downstream LSR is optional, because this information from the downstream LSR is often not available on the responder LSR. In such case, the traversal of LAG member links can be validated with procedures described in Section 5.1. If LSRs can provide the Remote Interface Index Sub- TLVs, then the validation procedures described in Section 5.2 can be used.5.1. Incoming LAG Member Links Verification
Without downstream LSRs returning Remote Interface Index Sub-TLVs in the DDMAP, validation of the LAG member link traversal requires that the initiator LSR traverses all available LAG member links and takes the results through additional logic. This section provides the mechanism for the initiator LSR to obtain additional information from the downstream LSRs and describes the additional logic in the initiator LSR to validate the L2 ECMP traversal.5.1.1. Initiator LSR Procedures
An MPLS echo request carrying a DDMAP TLV with the Interface and Label Stack Object Request flag and LAG Description Indicator flag set is sent to indicate the request for Detailed Interface and Label Stack TLV with additional LAG member link information (i.e., interface index) in the MPLS echo reply.
5.1.2. Responder LSR Procedures
When it receives an echo request with the LAG Description Indicator flag set, a responder LSR that understands that flag and is capable of describing the incoming LAG member link SHOULD use the following procedures, regardless of whether or not the incoming interface was a LAG interface: o When the I flag (Interface and Label Stack Object Request flag) of the DDMAP TLV in the received MPLS echo request is set: * The responder LSR MUST add the Detailed Interface and Label Stack TLV (described in Section 10) in the MPLS echo reply. * If the incoming interface is a LAG, the responder LSR MUST add the Incoming Interface Index Sub-TLV (described in Section 10.1.2) in the Detailed Interface and Label Stack TLV. The LAG Member Link Indicator flag MUST be set in the Interface Index Flags field, and the Interface Index field set to the LAG member link that received the MPLS echo request. These procedures allow the initiator LSR to utilize the Incoming Interface Index Sub-TLV in the Detailed Interface and the Label Stack TLV to derive, if the incoming interface is a LAG, the identity of the incoming LAG member.5.1.3. Additional Initiator LSR Procedures
Along with procedures described in Section 4, the procedures described in this section will allow an initiator LSR to know: o The expected load-balance information of every LAG member link, at LSR with TTL=n. o With specific entropy, the expected interface index of the outgoing LAG member link at TTL=n. o With specific entropy, the interface index of the incoming LAG member link at TTL=n+1. Depending on the LAG traffic division algorithm, the messages may or may not traverse different member links. The expectation is that there's a relationship between the interface index of the outgoing LAG member link at TTL=n and the interface index of the incoming LAG member link at TTL=n+1 for all entropies examined. In other words, the messages with a set of entropies that load-balances to outgoing LAG member link X at TTL=n should all reach the next hop on the same incoming LAG member link Y at TTL=n+1.
With additional logic, the initiator LSR can perform the following
checks in a scenario where it (a) knows that there is a LAG that has
two LAG members, between TTL=n and TTL=n+1, and (b) has the multipath
information to traverse the two LAG member links.
The initiator LSR sends two MPLS echo request messages to traverse
the two LAG member links at TTL=n+1:
o Success case:
* One MPLS echo request message reaches TTL=n+1 on LAG member
link 1.
* The other MPLS echo request message reaches TTL=n+1 on LAG
member link 2.
The two MPLS echo request messages sent by the initiator LSR reach
the immediate downstream LSR from two different LAG member links.
o Error case:
* One MPLS echo request message reaches TTL=n+1 on LAG member
link 1.
* The other MPLS echo request message also reaches TTL=n+1 on LAG
member link 1.
* One or both MPLS echo request messages cannot reach the
immediate downstream LSR on whichever link.
One or two MPLS echo request messages sent by the initiator LSR
cannot reach the immediate downstream LSR, or the two MPLS echo
request messages reach at the immediate downstream LSR from the
same LAG member link.
Note that the procedures defined above will provide a deterministic
result for LAG interfaces that are back-to-back connected between
LSRs (i.e., no L2 switch in between). If there is an L2 switch
between the LSR at TTL=n and the LSR at TTL=n+1, there is no
guarantee that every incoming interface at TTL=n+1 can be traversed,
even when traversing every outgoing LAG member link at TTL=n. Issues
resulting from LAG with an L2 switch in between are further described
in Appendix A. LAG provisioning models in operator networks should
be considered when analyzing the output of LSP Traceroute that is
exercising L2 ECMPs.
5.2. Individual End-to-End Path Verification
When the Remote Interface Index Sub-TLVs are available from an LSR with TTL=n, then the validation of LAG member link traversal can be performed by the downstream LSR of TTL=n+1. The initiator LSR follows the procedures described in Section 4.3. The DDMAP validation procedures for the downstream responder LSR are then updated to include the comparison of the incoming LAG member link to the interface index described in the Remote Interface Index Sub-TLV in the DDMAP TLV. Failure of this comparison results in the return code being set to "Downstream Mapping Mismatch (5)".6. LSR Capability TLV
This document defines a new TLV that is referred to as the LSR Capability TLV. It MAY be included in the MPLS echo request message and the MPLS echo reply message. An MPLS echo request message and an MPLS echo reply message MUST NOT include more than one LSR Capability TLV. The presence of an LSR Capability TLV in an MPLS echo request message is a request that a responder LSR includes an LSR Capability TLV in the MPLS echo reply message, with the LSR Capability TLV describing features and extensions that the responder LSR supports. The format of the LSR Capability TLV is as below: LSR Capability TLV Type is 4. Length is 4. The LSR Capability TLV 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LSR Capability Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: LSR Capability TLV Where: The Type field is 2 octets in length, and the value is 4. The Length field is 2 octets in length, and the value is 4.
The LSR Capability Flags field is 4 octets in length; this
document defines the following flags:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (Must Be Zero) |U|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document defines two flags. The unallocated flags MUST be
set to zero when sending and ignored on receipt. Both the U and
the D flag MUST be cleared in the MPLS echo request message when
sending and ignored on receipt. Zero, one, or both of the flags
(U and D) MAY be set in the MPLS echo reply message.
Flag Name and Meaning
---- ----------------
U Upstream LAG Info Accommodation
An LSR sets this flag when the LSR is capable of describing
a LAG member link in the Incoming Interface Index Sub-TLV
in the Detailed Interface and Label Stack TLV.
D Downstream LAG Info Accommodation
An LSR sets this flag when the LSR is capable of describing
LAG member links in the Local Interface Index Sub-TLV and
the Multipath Data Sub-TLV in the Downstream Detailed
Mapping TLV.
7. LAG Description Indicator Flag: G
This document defines a new flag, the G flag (LAG Description
Indicator), in the DS Flags field of the DDMAP TLV.
The G flag in the MPLS echo request message indicates the request for
detailed LAG information from the responder LSR. In the MPLS echo
reply message, the G flag MUST be set if the DDMAP TLV describes a
LAG interface. It MUST be cleared otherwise.
The G flag is defined as below:
The Bit Number is 3.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |G|E|L|I|N|
+-+-+-+-+-+-+-+-+
Flag Name and Meaning
---- ----------------
G LAG Description Indicator
When this flag is set in the MPLS echo request, the responder
LSR is requested to respond with detailed LAG information.
When this flag is set in the MPLS echo reply, the corresponding
DDMAP TLV describes a LAG interface.
8. Local Interface Index Sub-TLV
The Local Interface Index Sub-TLV describes the interface index
assigned by the local LSR to an egress interface. One or more Local
Interface Index sub-TLVs MAY appear in a DDMAP TLV.
The format of the Local Interface Index Sub-TLV is below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Local Interface Index Sub-TLV
Where:
o The Type field is 2 octets in length, and the value is 4.
o The Length field is 2 octets in length, and the value is 4.
o The Local Interface Index field is 4 octets in length; it is an
interface index assigned by a local LSR to an egress interface.
It's normally an unsigned integer and in network byte order.
9. Remote Interface Index Sub-TLV
The Remote Interface Index Sub-TLV is an optional TLV; it describes the interface index assigned by a downstream LSR to an ingress interface. One or more Remote Interface Index sub-TLVs MAY appear in a DDMAP TLV. The format of the Remote Interface Index Sub-TLV is below: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Interface Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Remote Interface Index Sub-TLV Where: o The Type field is 2 octets in length, and the value is 5. o The Length field is 2 octets in length, and the value is 4. o The Remote Interface Index field is 4 octets in length; it is an interface index assigned by a downstream LSR to an ingress interface. It's normally an unsigned integer and in network byte order.10. Detailed Interface and Label Stack TLV
The Detailed Interface and Label Stack TLV MAY be included in an MPLS echo reply message to report the interface on which the MPLS echo request message was received and the label stack that was on the packet when it was received. A responder LSR MUST NOT insert more than one instance of this TLV into the MPLS echo reply message. This TLV allows the initiator LSR to obtain the exact interface and label stack information as it appears at the responder LSR. Detailed Interface and Label Stack TLV Type is 6. Length is K + Sub- TLV Length (sum of Sub-TLVs). K is the sum of all fields of this TLV prior to the list of Sub-TLVs, but the length of K depends on the Address Type. Details of this information is described below. The Detailed Interface and Label Stack TLV 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Type | Reserved (Must Be Zero) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP Address (4 or 16 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface (4 or 16 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . List of Sub-TLVs . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Detailed Interface and Label Stack TLV The Detailed Interface and Label Stack TLV format is derived from the Interface and Label Stack TLV format (from [RFC8029]). Two changes are introduced. The first is that the label stack is converted into a sub-TLV. The second is that a new sub-TLV is added to describe an interface index. The other fields of the Detailed Interface and Label Stack TLV have the same use and meaning as in [RFC8029]. A summary of these fields is as below: Address Type The Address Type indicates if the interface is numbered or unnumbered. It also determines the length of the IP Address and Interface fields. The resulting total length of the initial part of the TLV is listed as "K Octets". The Address Type is set to one of the following values: Type # Address Type K Octets ------ ------------ -------- 1 IPv4 Numbered 16 2 IPv4 Unnumbered 16 3 IPv6 Numbered 40 4 IPv6 Unnumbered 28 IP Address and Interface IPv4 addresses and interface indices are encoded in 4 octets; IPv6 addresses are encoded in 16 octets. If the interface upon which the echo request message was received is numbered, then the Address Type MUST be set to IPv4
Numbered or IPv6 Numbered, the IP Address MUST be set to either
the LSR's Router ID or the interface address, and the Interface
MUST be set to the interface address.
If the interface is unnumbered, the Address Type MUST be either
IPv4 Unnumbered or IPv6 Unnumbered, the IP Address MUST be the
LSR's Router ID, and the Interface MUST be set to the index
assigned to the interface.
Note: Usage of IPv6 Unnumbered has the same issue as [RFC8029],
which is described in Section 3.4.2 of [RFC7439]. A solution
should be considered and applied to both [RFC8029] and this
document.
10.1. Sub-TLVs
This section defines the sub-TLVs that MAY be included as part of the
Detailed Interface and Label Stack TLV. Two sub-TLVs are defined:
Sub-Type Sub-TLV Name
--------- ------------
1 Incoming Label Stack
2 Incoming Interface Index
10.1.1. Incoming Label Stack Sub-TLV
The Incoming Label Stack Sub-TLV contains the label stack as received
by an LSR. If any TTL values have been changed by this LSR, they
SHOULD be restored.
Incoming Label Stack Sub-TLV Type is 1. Length is variable, and its
format is as below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Incoming Label Stack Sub-TLV
10.1.2. Incoming Interface Index Sub-TLV
The Incoming Interface Index Sub-TLV MAY be included in a Detailed Interface and Label Stack TLV. The Incoming Interface Index Sub-TLV describes the index assigned by a local LSR to the interface that received the MPLS echo request message. Incoming Interface Index Sub-TLV Type is 2. Length is 8, and its format is as below: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Index Flags | Reserved (Must Be Zero) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Incoming Interface Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Incoming Interface Index Sub-TLV Interface Index Flags The Interface Index Flags field is a bit vector with following format. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved (Must Be Zero) |M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ One flag is defined: M. The remaining flags MUST be set to zero when sending and ignored on receipt. Flag Name and Meaning ---- ---------------- M LAG Member Link Indicator When this flag is set, the interface index described in this sub-TLV is a member of a LAG. Incoming Interface Index An Index assigned by the LSR to this interface. It's normally an unsigned integer and in network byte order.
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