Network Working Group K. Kompella (Juniper)
Internet Draft P. Pan (Ciena)
draft-ietf-mpls-lsp-ping-01.txt N. Sheth (Juniper)
Category: Standards Track D. Cooper (Global Crossing)
Expires: April 2003 G. Swallow (Cisco)
S. Wadhwa (Juniper)
R. Bonica (WorldCom)
October 2002
Detecting MPLS Data Plane Liveness
*** DRAFT ***
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Abstract
This document describes a simple and efficient mechanism that can be
used to detect data plane failures in Multi-Protocol Label Switching
(MPLS) Label Switched Paths (LSPs). There are two parts to this
document: information carried in an MPLS "echo request" and "echo
reply" for the purposes of fault detection and isolation; and
mechanisms for reliably sending the echo reply.
Sub-IP ID Summary
(This section to be removed before publication.)
(See Abstract above.)
RELATED DOCUMENTS
May be found in the "references" section.
WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
Fits in the MPLS box.
WHY IS IT TARGETED AT THIS WG
MPLS WG is currently looking at MPLS-specific error detection and
recovery mechanisms. The mechanisms proposed here are for packet-
based MPLS LSPs, which is why the MPLS WG is targeted.
JUSTIFICATION
The WG should consider this document, as it allows network operators
to detect MPLS LSP data plane failures in the network. This type of
failures have occurred, and are a source of concern to operators
implementing MPLS networks.
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1. Introduction
This document describes a simple and efficient mechanism that can be
used to detect data plane failures in MPLS LSPs. There are two parts
to this document: information carried in an MPLS "echo request" and
"echo reply"; and mechanisms for transporting the echo reply. The
first part aims at providing enough information to check correct
operation of the data plane, as well as a mechanism to verify the
data plane against the control plane, and thereby localize faults.
The second part suggests two methods of reliable reply channels for
the echo request message, for more robust fault isolation.
An important consideration in this design is that MPLS echo requests
follow the same data path that normal MPLS packets would traverse.
MPLS echo requests are meant primarily to validate the data plane,
and secondarily to verify the data plane against the control plane.
Mechanisms to check the control plane are valuable, but are not
covered in this document.
To avoid potential Denial of Service attacks, it is recommended to
regulate the MPLS ping traffic going to the control plane. A rate
limiter should be applied to the well-known UDP port defined below.
1.1. Conventions
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 [KEYWORDS].
1.2. Changes since last revision
(This section to be removed before publication.)
- Packet format changed; Version Number field added
- Reply modes: "don't reply" added
- Reply flags removed
- Return codes extended
- RSVP session formats modified
- VPN IPv4/v6 formats defined
- L2 VPN endpoint and L2 circuits defined
- Downstream mapping format changed
- Pad and Error Code TLVs introduced
- Aspects dealing with CR-LDP moved to non-normative appendix
- IPR notices and Full Copyright Statement (per 2026) added
- other nits to better conform to 2223bis
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2. Motivation
When an LSP fails to deliver user traffic, the failure cannot always
be detected by the MPLS control plane. There is a need to provide a
tool that would enable users to detect such traffic "black holes" or
misrouting within a reasonable period of time; and a mechanism to
isolate faults.
In this document, we describe a mechanism that accomplishes these
goals. This mechanism is modeled after the ping/traceroute
philosophy: ping (ICMP echo request [ICMP]) is used for connectivity
checks, and traceroute is used for hop-by-hop fault localization as
well as path tracing. This document specifies a "ping mode" and a
"traceroute" mode for testing MPLS LSPs.
The basic idea is to test that packets that belong to a particular
Forwarding Equivalence Class (FEC) actually end their MPLS path on an
LSR that is an egress for that FEC. This document proposes that this
test be carried out by sending a packet (called an "MPLS echo
request") along the same data path as other packets belonging to this
FEC. An MPLS echo request also carries information about the FEC
whose MPLS path is being verified. This echo request is forwarded
just like any other packet belonging to that FEC. In "ping" mode
(basic connectivity check), the packet should reach the end of the
path, at which point it is sent to the control plane of the egress
LSR, which then verifies that it is indeed an egress for the FEC. In
"traceroute" mode (fault isolation), the packet is sent to the
control plane of each transit LSR, which performs various checks that
it is indeed a transit LSR for this path; this LSR also returns
further information that helps check the control plane against the
data plane, i.e., that forwarding matches what the routing protocols
determined as the path.
One way these tools can be used is to periodically ping a FEC to
ensure connectivity. If the ping fails, one can then initiate a
traceroute to determine where the fault lies. One can also
periodically traceroute FECs to verify that forwarding matches the
control plane; however, this places a greater burden on transit LSRs
and thus should be used with caution.
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3. Packet Format
An MPLS echo request is a (possibly labelled) UDP packet; the
contents of the UDP packet have 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Number | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reply mode | Return Code | Return Subcode|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs ... |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Version Number is currently 1. (Note: the Version Number is to
be incremented whenever a change is made that affects the ability of
an implementation to correctly parse or process an MPLS echo
request/reply. These changes include any syntactic or semantic
changes made to any of the fixed fields, or to any TLV or sub-TLV
assignment or format that is defined at a certain version number.
The Version Number may not need to be changed if a TLV or sub-TLV is
added.)
The Message Type is one of the following:
Value Meaning
----- -------
1 MPLS Echo Request
2 MPLS Echo Reply
The Reply Mode can take one of the following values:
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Value Meaning
----- -------
1 Do not reply
2 Reply via an IPv4 UDP packet
3 Reply via an IPv4 UDP packet with Router Alert
4 Reply via the control plane
An MPLS echo request with "Do not reply" may be used for one-way
connectivity tests; the receiving router may log gaps in the sequence
numbers and/or maintain delay/jitter statistics. An MPLS echo
request would normally have "Reply via an IPv4 UDP packet"; if the
normal IPv4 return path is deemed unreliable, one may use "Reply via
an IPv4 UDP packet with Router Alert" (note that this requires that
all intermediate routers understand and know how to forward MPLS echo
replies) or "Reply via the control plane" (this is currently only
defined for control plane that uses RSVP).
The Return Code is set to zero by the sender. The receiver can set
it to one of the following values:
Value Meaning
----- -------
0 The error code is contained in the Error Code TLV
1 Malformed echo request received
2 One or more of the TLVs was not understood
3 Replying router is an egress for the FEC
4 Replying router has no mapping for the FEC
5 Replying router is not one of the "Downstream Routers"
6 Replying router is one of the "Downstream Routers",
and its mapping for this FEC on the received interface
is the given label
7 Replying router is one of the "Downstream Routers",
but its mapping for this FEC is not the given label
The Return Subcode is unused at present and SHOULD be set to zero.
The Sender's Handle is filled in by the sender, and returned
unchanged by the receiver in the echo reply (if any). There are no
semantics associated with this handle, although a sender may find
this useful for matching up requests with replies.
The Sequence Number is assigned by the sender of the MPLS echo
request, and can be (for example) used to detect missed replies.
The TimeStamp Sent is the time-of-day (in seconds and microseconds,
wrt the sender's clock) when the MPLS echo request is sent. The
TimeStamp Received in an echo reply is the time-of-day (wrt the
receiver's clock) that the corresponding echo request was received.
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TLVs (Type-Length-Value tuples) have 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Types are defined below; Length is the length of the Value field in
octets. The Value field depends on the Type; it is zero padded to
align to a four-octet boundary.
Type # Value Field
------ -----------
1 Target FEC Stack
2 Downstream Mapping
3 Pad
4 Error Code
3.1. Target FEC Stack
A Target FEC Stack is a list of sub-TLVs. The number of elements is
determined by the looking at the sub-TLV length fields.
Sub-Type # Length Value Field
---------- ------ -----------
1 5 LDP IPv4 prefix
2 17 LDP IPv6 prefix
3 20 RSVP IPv4 Session Query
4 56 RSVP IPv6 Session Query
5 Reserved; see Appendix
6 13 VPN IPv4 prefix
7 25 VPN IPv6 prefix
8 14 L2 VPN endpoint
9 10 L2 circuit ID
Other FEC Types will be defined as needed.
Note that this TLV defines a stack of FECs, the first FEC element
corresponding to the top of the label stack, etc.
An MPLS echo request MUST have a Target FEC Stack that describes the
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FEC stack being tested. For example, if an LSR X has an LDP mapping
for 192.168.1.1 (say label 1001), then to verify that label 1001 does
indeed reach an egress LSR that announced this prefix via LDP, X can
send an MPLS echo request with a FEC Stack TLV with one FEC in it,
namely of type LDP IPv4 prefix, with prefix 192.168.1.1/32, and send
the echo request with a label of 1001.
If LSR X wanted to verify that a label stack of <1001, 23456> is the
right label stack to use to reach an IP VPN prefix of 10/8 in VPN foo
on an egress LSR with loopback address 192.168.1.1 (learned via LDP),
X has two choices. X can send an MPLS echo request with a FEC Stack
TLV with a single FEC of type VPN IPv4 prefix with a prefix of 10/8
with the Route Distinguisher for VPN foo. Alternatively, X can send
a FEC Stack TLV with two FECs, the first of type LDP IPv4 with a
prefix of 192.168.1.1/32 and the second of type of IP VPN with a
prefix 10/8 in VPN foo. In either case, the MPLS echo request would
have a label stack of <1001, 23456>. (Note: in this example, 1001 is
the "outer" label and 23456 is the "inner" label.)
3.1.1. IPv4 Prefix
The value consists of four octets of an IPv4 prefix followed by one
octet of prefix length in bits. The IPv4 prefix is in network byte
order. See [LDP] for an example of a Mapping for an IPv4 FEC.
3.1.2. IPv6 Prefix
The value consists of sixteen octets of an IPv6 prefix followed by
one octet of prefix length in bits. The IPv6 prefix is in network
byte order.
3.1.3. RSVP IPv4 Session
The value has the format below. The value fields are taken from
[RFC3209, sections 4.6.1.1 and 4.6.2.1].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel end point address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel sender address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | LSP ID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.4. RSVP IPv6 Session
The value has the format below. The value fields are taken from
[RFC3209, sections 4.6.1.2 and 4.6.2.2].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel end point address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel sender address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | LSP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.5. VPN IPv4 Prefix
The value field consists of a Route Distinguisher, an IPv4 prefix and
a prefix length, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.1.6. VPN IPv6 Prefix
The value field consists of a Route Distinguisher, an IPv6 prefix and
a prefix length, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 prefix |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.7. L2 VPN Endpoint
The value field consists of a Route Distinguisher (8 octets), the
sender (of the ping)'s CE ID (2 octets), the receiver's CE ID (2
octets), and an encapsulation type (2 octets), formatted 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's CE ID | Receiver's CE ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encapsulation Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.8. L2 Circuit ID
The value field consists of a remote PE address (the address of the
targetted LDP session), a VC ID and an encapsulation type, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote PE Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VC ID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encapsulation Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2. Downstream Mapping
The Downstream Mapping is an optional TLV in an echo request. The
Length is 12 + 4*N octets, where N is the number of Downstream
Labels. The Value of a Downstream Mapping 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream IPv4 Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Address Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MTU is the largest MPLS frame (including label stack) that fits
on the interface to the Downstream LSR. The Address Type is one of:
Type # Address Type
------ ------------
1 IPv4
2 Unnumbered
'Protocol' is taken from the following table:
Protocol # Signaling Protocol
---------- ------------------
0 Unknown
1 Static
2 BGP
3 LDP
4 RSVP-TE
5 Reserved; see Appendix
The notion of "downstream router" should be explained. Consider an
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LSR X. If a packet with outermost label L and TTL n>1 arrived at X
on interface I, X must be able to compute which LSRs could receive
the packet with TTL=n+1, and what label they would see. (It is
outside the scope of this document to specify how this computation
may be done.) The set of these LSRs are the downstream routers (and
their corresponding labels) for X with respect to L.
The case where X is the LSR originating the echo request is a special
case. X needs to figure out what LSRs would receive a labelled
packet with TTL=1 when X tries to send a packet to the FEC Stack that
is being pinged.
3.3. Pad TLV
The value part of the Pad TLV contains a variable number (>= 1) of
octets. The first octet takes values from the following table; all
the other octets (if any) are ignored. The receiver SHOULD verify
that the TLV is received in its entirety, but otherwise ignores the
contents of this TLV, apart from the first octet.
Value Meaning
----- -------
1 Drop Pad TLV from reply
2 Copy Pad TLV to reply
3-255 Reserved for future use
3.4. Error Code
The Error Code TLV is currently not defined; its purpose is to
provide a mechanism for a more elaborate error reporting structure,
should the reason arise.
4. Theory of Operation
4.1. Sending an MPLS Echo Request
An MPLS echo request is a (possibly) labelled UDP packet. The IP
header is set as follows: the source IP address is a routable address
of the sender; the destination IP address is a (randomly chosen)
address from 127/8; the IP TTL is set to 1. The source UDP port is
chosen by the sender; the destination UDP port is set to 3503
(assigned by IANA for MPLS echo requests). If the echo request is
labelled, the MPLS TTL on all the labels except the outermost should
be set to 1.
In "ping" mode (end-to-end connectivity check), the TTL in the
outermost label is set to 255. In "traceroute" mode (fault isolation
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mode), the TTL is set successively to 1, 2, ....
The sender chooses a Sender's Handle, and a Sequence Number. When
sending subsequent MPLS echo requests, the sender SHOULD increment
the sequence number by 1. However, a sender MAY choose to send a
group of echo requests with the same sequence number to improve the
chance of arrival of at least one packet with that sequence number.
The TimeStamp Sent is set to the time-of-day (in seconds and
microseconds) that the echo request is sent. The TimeStamp Received
is set to zero.
An MPLS echo request MUST have a FEC Stack TLV. Also, the Reply Mode
must be set to the desired reply mode; the Return Code and Subcode
are set to zero.
In the "traceroute" mode, the echo request SHOULD contain one or more
Downstream Mapping TLVs. For TTL=1, all the downstream routers (and
corresponding labels) for the sender with respect to the FEC Stack
being pinged SHOULD be sent in the echo request. For n>1, the
Downstream Mapping TLVs from the echo reply for TTL=(n-1) are copied
to the echo request with TTL=n.
4.2. Receiving an MPLS Echo Request
An LSR L that receives an MPLS echo request first parses the packet
to ensure that it is a well-formed packet, and that the TLVs are
understood. If not, L SHOULD send an MPLS echo reply with the
Return Code set to "Malformed echo request received" or "TLV not
understood" (as appropriate), and the Subcode set to the appropriate
value.
If the echo request is good, L then checks whether it is a valid
transit or egress LSR for the FEC in the echo request. If not, L MAY
log this fact.
If the echo request contains a Downstream Mapping TLV, L MUST further
check whether its Router ID matches one of the Downstream IPv4 Router
IDs; and if so, whether the given Downstream Label is in fact the
label that L sent as its mapping for the FEC. For an RSVP FEC, the
downstream label is the label that L sent in its Resv message. The
result of the checks in the previous and this paragraph are captured
in the Return Code/Subcode.
If the echo request has a Reply Mode that wants a reply, L uses the
procedure in the next subsection to send the echo reply.
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4.3. Sending an MPLS Echo Reply
An MPLS echo reply is a UDP packet. It MUST ONLY be sent in response
to an MPLS echo request. The source IP address is a routable address
of the replier; the source port is the well-known UDP port for MPLS
ping. The destination IP address and UDP port are copied from the
source IP address and UDP port of the echo request. The IP TTL is
set to 255. If the Reply Mode in the echo request is "Reply via an
IPv4 UDP packet with Router Alert", then the IP header MUST contain
the Router Alert IP option.
The format of the echo reply is the same as the echo request. The
Sender's Handle, the Sequence Number and TimeStamp Sent are copied
from the echo request; the TimeStamp Received is set to the time-of-
day that the echo request is received (note that this information is
most useful if the time-of-day clocks on the requestor and the
replier are synchronized). The FEC Stack TLV from the echo request
MAY be copied to the reply.
The replier MUST fill in the Return Code and Subcode, as determined
in the previous subsection.
If the echo request contains a Pad TLV, the replier MUST interpret
the first octet for instructions regarding how to reply.
If the echo request contains a Downstream Mapping TLV, the replier
SHOULD compute its downstream routers and corresponding labels for
the incoming label, and add Downstream Mapping TLVs for each one to
the echo reply it sends back.
4.4. Receiving an MPLS Echo Reply
An LSR X should only receive an MPLS Echo Reply in response to an
MPLS Echo Request that it sent. Thus, on receipt of an MPLS Echo
Reply, X should parse the packet to assure that it is well-formed,
then attempt to match up the Echo Reply with an Echo Request that it
had previously sent, using the destination UDP port and the Sender's
Handle. If no match is found, then X jettisons the Echo Reply;
otherwise, it checks the Sequence Number to see if it matches. Gaps
in the Sequence Number MAY be logged and SHOULD be counted. Once an
Echo Reply is received for a given Sequence Number (for a given UDP
port and Handle), the Sequence Number for subsequent Echo Requests
for that UDP port and Handle SHOULD be incremented.
If the Echo Reply contains Downstream Mappings, and X wishes to
traceroute further, it SHOULD copy the Downstream Mappings into its
next Echo Request (with TTL incremented by one).
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4.5. Non-compliant Routers
If the egress for the FEC Stack being pinged does not support MPLS
ping, then no reply will be sent, resulting in possible "false
negatives". If in "traceroute" mode, a transit LSR does not support
MPLS ping, then no reply will be forthcoming from that LSR for some
TTL, say n. The LSR originating the echo request SHOULD try sending
the echo request with TTL=n+1, n+2, ..., n+k in the hope that some
transit LSR further downstream may support MPLS echo requests and
reply. In such a case, the echo request for TTL>n MUST NOT have
Downstream Mapping TLVs, until a reply is received with a Downstream
Mapping.
5. Reliable Reply Path
One of the issues that are faced with MPLS ping is to distinguish
between a failure in the forward path (the MPLS path being 'pinged')
and a failure in the return path. Note that this problem exists with
vanilla IP ping as well. In the case of MPLS ping, it is assumed
that the IP control and data planes are reliable. However, it could
be that the forwarding in the return path is via an MPLS LSP.
In this specification, we give two solutions for this problem. One
is to set the Router Alert option in the MPLS echo reply. When a
router sees this option, it MUST forward the packet as an IP packet.
Note that this may not work if some transit LSR does not support MPLS
ping.
Another option is to send the echo reply via the control plane. At
present, this is defined only for RSVP-TE LSPs, and described below.
These options are controlled by the ingress LSR, using the Reply Mode
in the MPLS echo request packet.
5.1. RSVP-TE Extension
To test an LSP's liveliness, an ingress LSR sends MPLS echo requests
over the LSP being tested. When an egress LSR receives the message,
it needs to acknowledge the ingress LSR by sending an LSP_ECHO object
in a RSVP Resv message. The object has the following format:
Class = LSP_ECHO (use form 11bbbbbb for compatibility)
C-Type = 1
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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp (microseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Source Port | Return Code | Return Subcode|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Sequence Number is copied from the Sequence Number of the echo
request. The TimeStamp is set to the time the echo request is
received. The UDP Source Port is copied from the UDP source port of
the MPLS echo request. The FEC is implied by the Session and the
Sender Template Objects.
5.2. Operation
For the sake of brevity in the context of this document by "the
control plane" we mean "the RSVP-TE component of the control plane".
Consider an LSP between an ingress LSR and an egress LSR spanning
multiple LSR hops.
5.3. Procedures at the ingress LSR
One must ensure before setting the Reply Mode to "reply via the
control plane" that the egress LSR supports this feature.
The ingress LSR, say X, builds an MPLS echo request as in section
"Sending an MPLS Echo Request". The FEC Type must be an RVSP Session
Query. X also sets the Reply Mode to "reply via the control plane".
If X does not receive an Resv message from the egress LSR that
contains an LSP_ECHO object within some period of time, it declares
the LSP as "down". At this point, the ingress LSR may apply the
necessary procedures to fix the LSP. These may include generating a
message to network management, tearing-down and re-building the LSP,
and/or rerouting user traffic to a backup LSP.
To test an LSP that carries non-IP traffic, before injecting ICMP and
MPLS ping messages into the LSP, the IPv4 Explicit NULL label should
be prepended to such messages. The ingress and egress LSR's must
follow the procedures defined in [LABEL-STACKING].
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5.4. Procedures at the egress LSR
When the egress LSR receives an MPLS ping message, it follows the
procedures given above. If the Reply Mode is set to "Reply via the
control plane", the LSR can, based on the RSVP SESSION and
SENDER_TEMPLATE objects carried in the MPLS ping message, find the
corresponding LSP in its RSVP-TE database. The LSR then checks to
see if the Resv message for this LSP contains an LSP_ECHO object with
the same source UDP port value. If not, the LSR adds or updates the
LSP_ECHO object and refreshes the Resv message.
5.5. Procedures for the intermediate LSR's
At intermediate LSRs, normal RSVP processing procedures will cause
the LSP_ECHO object to be forwarded as RSVP messages are refreshed.
At the LSR's that support MPLS ping the Resv messages that carry the
LSP_ECHO object MUST be delivered upstream immediately.
Note that an intermediate LSR using RSVP refresh reduction [RSVP-
REFRESH], the new or changed LSP_ECHO object will cause the LSR to
classify the RSVP message as a trigger message.
6. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LABEL-STACKING] Rosen, E., et al, "MPLS Label Stack Encoding", RFC
3032, January 2001.
[RSVP] Braden, R. (Editor), et al, "Resource ReSerVation protocol
(RSVP) -- Version 1 Functional Specification," RFC 2205,
September 1997.
[RSVP-REFRESH] Berger, L., et al, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RSVP-TE] Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC 3209, December 2001.
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7. Informative References
[ICMP] Postel, J., "Internet Control Message Protocol", RFC 792.
[LDP] Andersson, L., et al, "LDP Specification", RFC 3036, January
2001.
8. Security Considerations
There are at least two approaches to attacking LSRs using the
mechanisms defined here. One is a Denial of Service attack, by
sending MPLS echo requests/replies to LSRs and thereby increasing
their workload. The other is obfuscating the state of the MPLS data
plane liveness by spoofing, hijacking, replaying or otherwise
tampering with MPLS echo requests and replies.
Authentication will help reduce the number of seemingly valid MPLS
echo requests, and thus cut down the Denial of Service attacks;
beyond that, each LSR must protect itself.
Authentication sufficiently addresses spoofing, replay and most
tampering attacks; one hopes to use some mechanism devised or
suggested by the RPSec WG. It is not clear how to prevent hijacking
(non-delivery) of echo requests or replies; however, if these
messages are indeed hijacked, MPLS ping will report that the data
plane isn't working as it should.
It doesn't seem vital (at this point) to secure the data carried in
MPLS echo requests and replies, although knowledge of the state of
the MPLS data plane may be considered confidential by some.
9. IANA Considerations
(To be filled in a later revision)
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10. Acknowledgments
This document is the outcome of many discussions among many people,
that include Manoj Leelanivas, Paul Traina, Yakov Rekhter, Der-Hwa
Gan, Brook Bailey, Eric Rosen and Ina Minei.
11. Appendix
This appendix specifies non-normative aspects of detecting MPLS data
plane liveness.
11.1. CR-LDP FEC
This section describes how a CR-LDP FEC can be included in an Echo
Request using the following FEC subtype:
Sub-Type # Length Value Field
---------- ------ -----------
5 6 CR-LDP LSP ID
The value consists of the LSPID of the LSP being pinged. An LSPID is
a four octet IPv4 address (a local address on the ingress LSR, for
example, the Router ID) plus a two octet identifier that is unique
per LSP on a given ingress LSR.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ingress LSR Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | LSP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
11.2. Downstream Mapping for CR-LDP
If a label in a Downstream Mapping was learned via CR-LDP, the
Protocol field in the Mapping TLV can use the following entry:
Protocol # Signaling Protocol
---------- ------------------
5 CR-LDP
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12. Authors' Addresses
Kireeti Kompella
Nischal Sheth
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
e-mail: nsheth@juniper.net
Ping Pan
Ciena
10480 Ridgeview Court
Cupertino, CA 95014
e-mail: ppan@ciena.com
phone: +1 408.366.4700
Dave Cooper
Global Crossing
960 Hamlin Court
Sunnyvale, CA 94089
email: dcooper@gblx.net
phone: +1 916.415.0437
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
e-mail: swallow@cisco.com
phone: +1 978.497.8143
Sanjay Wadhwa
Juniper Networks
10 Technology Park Drive
Westford, MA 01886-3146
email: swadhwa@unispherenetworks.com
phone: +1 978.589.0697
Ronald P. Bonica
WorldCom
22001 Loudoun County Pkwy
Ashburn, Virginia, 20147
email: ronald.p.bonica@wcom.com
phone: +1 703.886.1681
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13. Intellectual Property Rights Notices
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The IETF invites any interested party to bring to its attention any
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Kompella et al Standards Track [Page 22]
