Label Switched Path (LSP) Self-Ping
A lightweight probe an ingress router sends to itself through a new MPLS path to check that every hop is ready before traffic flows.
Bonica, R., Minei, I., Conn, M., Pacella, D., and L. Tomotaki, "Label Switched Path (LSP) Self-Ping", RFC 7746, DOI 10.17487/RFC7746, January 2016, <https://www.rfc-editor.org/info/rfc7746>.
@misc{rfc7746,
series = {Request for Comments},
number = 7746,
howpublished = {RFC 7746},
publisher = {RFC Editor},
doi = {10.17487/RFC7746},
url = {https://www.rfc-editor.org/info/rfc7746},
author = {R. Bonica and I. Minei and M. Conn and D. Pacella and L. Tomotaki},
title = {{Label Switched Path (LSP) Self-Ping}},
pagetotal = 12,
year = 2016,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Bonica
Request for Comments: 7746 Juniper Networks
Category: Standards Track I. Minei
ISSN: 2070-1721 Google, Inc.
M. Conn
D. Pacella
L. Tomotaki
Verizon
January 2016
Label Switched Path (LSP) Self-Ping
Abstract
When certain RSVP-TE optimizations are implemented, ingress Label
Switching Router (LSRs) can receive RSVP RESV messages before
forwarding state has been installed on all downstream nodes.
According to the RSVP-TE specification, the ingress LSR can forward
traffic through a Label Switched Path (LSP) as soon as it receives a
RESV message. However, if the ingress LSR forwards traffic through
the LSP before forwarding state has been installed on all downstream
nodes, traffic can be lost.
This document describes LSP Self-ping. When an ingress LSR receives
an RESV message, it can invoke LSP Self-ping procedures to ensure
that forwarding state has been installed on all downstream nodes.
LSP Self-ping is a new protocol. It is not an extension of LSP Ping.
Although LSP Ping and LSP Self-ping are named similarly, each is
designed for a unique purpose. Each protocol listens on its own UDP
port and executes its own procedures.
LSP Self-ping is an extremely lightweight mechanism. It does not
consume control-plane resources on transit or egress LSRs.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7746.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The LSP Self-ping Message . . . . . . . . . . . . . . . . . . 6
4. LSP Self-Ping Procedures . . . . . . . . . . . . . . . . . . 7
5. Bidirectional LSP Procedures . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Rejected Approaches . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Ingress Label Switching Routers (LSRs) use RSVP-TE [RFC3209] to
establish MPLS Label Switched Paths (LSPs). The following paragraphs
describe RSVP-TE procedures.
The ingress LSR calculates a path between itself and an egress LSR.
The calculated path can be either strictly or loosely routed. Having
calculated a path, the ingress LSR constructs an RSVP PATH message.
The PATH message includes an Explicit Route Object (ERO) that
represents the path between the ingress and egress LSRs.
The ingress LSR forwards the PATH message towards the egress LSR,
following the path defined by the ERO. Each transit LSR that
receives the PATH message executes admission control procedures. If
the transit LSR admits the LSP, it sends the PATH message downstream,
to the next node in the ERO.
When the egress LSR receives the PATH message, it binds a label to
the LSP. The label can be implicit null, explicit null, or non-null.
The egress LSR then installs forwarding state (if necessary) and
constructs an RSVP RESV message. The RESV message contains a Label
Object that includes the label that has been bound to the LSP.
The egress LSR sends the RESV message upstream towards the ingress
LSR. The RESV message visits the same transit LSRs that the PATH
message visited, in reverse order. Each transit LSR binds a label to
the LSP, updates its forwarding state, and updates the RESV message.
As a result, the Label Object in the RESV message contains the label
that has been bound to the LSP most recently. Finally, the transit
LSR sends the RESV message upstream, along the reverse path of the
LSP.
When the ingress LSR receives the RESV message, it installs
forwarding state. Once the ingress LSR installs forwarding state, it
can forward traffic through the LSP.
Referring to any LSR, RFC 3209 says, "The node SHOULD be prepared to
forward packets carrying the assigned label prior to sending the Resv
message." However, RFC 3209 does not strictly require this behavior.
Some implementations optimize the above-described procedure by
allowing LSRs to send RESV messages before installing forwarding
state [RFC6383]. This optimization is desirable, because it allows
LSRs to install forwarding state in parallel, thus accelerating the
process of LSP signaling and setup. However, this optimization
creates a race condition. When the ingress LSR receives a RESV
message, some downstream LSRs may have not yet installed forwarding
state. If the ingress LSR forwards traffic through the LSP before
forwarding state has been installed on all downstream nodes, traffic
can be lost.
This document describes LSP Self-ping. When an ingress LSR receives
an RESV message, it can invoke LSP Self-ping procedures to verify
that forwarding state has been installed on all downstream nodes. By
verifying the installation of downstream forwarding state, the
ingress LSR eliminates this particular cause of traffic loss.
LSP Self-ping is a new protocol. It is not an extension of LSP Ping
[RFC4379]. Although LSP Ping and LSP Self-ping are named similarly,
each is designed for a unique purpose. Each protocol listens on its
own UDP port and executes its own procedures.
LSP Self-ping is an extremely lightweight mechanism. It does not
consume control-plane resources on transit or egress LSRs.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Applicability
LSP Self-ping is applicable in the following scenario:
o The ingress LSR signals a point-to-point LSP.
o The ingress LSR receives a RESV message.
o The RESV message indicates that all downstream nodes have begun
the process of forwarding state installation.
o The RESV message does not guarantee that all downstream nodes have
completed the process of forwarding state installation.
o The ingress LSR needs to confirm that all downstream nodes have
completed the process for forwarding state installation.
o The ingress LSR does not need to confirm the correctness of
downstream forwarding state, because there is a very high
likelihood that downstream forwarding state is correct.
o Control-plane resources on the egress LSR may be scarce.
o The need to conserve control-plane resources on the egress LSR
outweighs the need to determine whether downstream forwarding
state is correct.
Unlike LSP Ping and S-BFD [S-BFD], LSP Self-ping is not a general-
purpose MPLS OAM mechanism. It cannot reliably determine whether
downstream forwarding state is correct. For example, if a downstream
LSR installs a forwarding state that causes an LSP to terminate at
the wrong node, LSP Self-ping will not detect an error. In another
example, if a downstream LSR erroneously forwards a packet without an
MPLS label, LSP Self-ping will not detect an error.
Furthermore, LSP Self-ping fails when either of the following
conditions are true:
o The LSP under test is signaled by the Label Distribution Protocol
(LDP) Independent Mode [RFC5036].
o Reverse Path Forwarding (RPF) [RFC3704] filters are enabled on
links that connect the ingress LSR to the egress LSR.
While LSP Ping and S-BFD are general-purpose OAM mechanisms, they are
not applicable in the above-described scenario because:
o LSP Ping consumes control-plane resources on the egress LSR.
o An S-BFD implementation either consumes control-plane resources on
the egress LSR or requires special support for S-BFD on the
forwarding plane.
By contrast, LSP Self-ping requires nothing from the egress LSR
beyond the ability to forward an IP datagram.
LSP Self-ping's purpose is to determine whether forwarding state has
been installed on all downstream LSRs. Its primary constraint is to
minimize its impact on egress LSR performance. This functionality is
valuable during network convergence events that impact a large number
of LSPs.
Therefore, LSP Self-ping is applicable in the scenario described
above, where the LSP is signaled by RSVP, RPF is not enabled, and the
need to conserve control-plane resources on the egress LSR outweighs
the need to determine whether downstream forwarding state is correct.
3. The LSP Self-ping Message
The LSP Self-ping Message is a User Datagram Protocol (UDP) [RFC768]
packet that encapsulates a session ID. If the RSVP messages used to
establish the LSP under test were delivered over IPv4 [RFC791], the
UDP datagram MUST be encapsulated in an IPv4 header. If the RSVP
messages used to establish the LSP were delivered over IPv6
[RFC2460], the UDP datagram MUST be encapsulated in an IPv6 header.
In either case:
o The IP Source Address MAY be configurable. By default, it MUST be
the address of the egress LSR.
o The IP Destination Address MUST be the address of the ingress LSR.
o The IP Time to Live (TTL) / Hop Count MAY be configurable. By
default, it MUST be 255.
o The IP DSCP (Differentiated Services Code Point) MAY be
configurable. By default, it MUST be CS6 (110000) [RFC4594].
o The UDP Source Port MUST be selected from the dynamic range
(49152-65535) [RFC6335].
o The UDP Destination Port MUST be lsp-self-ping (8503) [IANA.PORTS]
UDP packet contents 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session-ID |
| (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LSP Self-Ping Message
The Session-ID is a 64-bit field that associates an LSP Self-ping
message with an LSP Self-ping session.
4. LSP Self-Ping Procedures
In order to verify that an LSP is ready to carry traffic, the ingress
LSR creates a short-lived LSP Self-ping session. All session state
is maintained locally on the ingress LSR. Session state includes the
following information:
o Session-ID: A 64-bit number that identifies the LSP Self-ping
session.
o Retry Counter: The maximum number of times that the ingress LSR
probes the LSP before terminating the LSP Self-ping session. The
initial value of this variable is determined by configuration.
o Retry Timer: The number of milliseconds that the LSR waits after
probing the LSP. The initial value of this variable is determined
by configuration.
o Status: A boolean variable indicating the completion status of the
LSP Self-ping session. The initial value of this variable is
FALSE.
Implementations MAY represent the above-mentioned information in any
format that is convenient to them.
The ingress LSR executes the following procedure until Status equals
TRUE or Retry Counter equals zero:
o Format a LSP Self-ping message.
o Set the Session-ID in the LSP Self-ping message to the Session-ID
mentioned above.
o Send the LSP Self-ping message through the LSP under test.
o Set a timer to expire in Retry Timer milliseconds.
o Wait until either an LSP Self-ping message associated with the
session returns or the timer expires. If an LSP Self-ping message
associated with the session returns, set Status to TRUE.
Otherwise, decrement the Retry Counter. Optionally, increase the
value of Retry Timer according to an appropriate back-off
algorithm.
In the process described above, the ingress LSR addresses an LSP
Self-ping message to itself and forwards that message through the LSP
under test. If forwarding state has been installed on all downstream
LSRs, the egress LSR receives the LSP Self-ping message and
determines that it is addressed to the ingress LSR. So, the egress
LSR forwards the LSP Self-ping message back to the ingress LSR,
exactly as it would forward any other IP packet.
The LSP Self-ping message can arrive at the egress LSR with or
without an MPLS header, depending on whether the LSP under test
executes penultimate hop-popping procedures. If the LSP Self-ping
message arrives at the egress LSR with an MPLS header, the egress LSR
removes that header.
If the egress LSR's most preferred route to the ingress LSR is
through an LSP, the egress LSR forwards the LSP Self-ping message
through that LSP. However, if the egress LSR's most preferred route
to the ingress LSR is not through an LSP, the egress LSR forwards the
LSP Self-ping message without MPLS encapsulation.
When an LSP Self-ping session terminates, it returns its completion
status to the invoking protocol. For example, if RSVP-TE invokes LSP
Self-ping as part of the LSP setup procedure, LSP Self-ping returns
its completion status to RSVP-TE.
5. Bidirectional LSP Procedures
A bidirectional LSP has an active side and a passive side. The
active side calculates the ERO and signals the LSP in the forward
direction. The passive side reverses the ERO and signals the LSP in
the reverse direction.
When LSP Self-ping is applied to a bidirectional LSP:
o The active side calculates the ERO, signals the LSP, and runs LSP
Self-ping.
o The Passive side reverses the ERO, signals the LSP, and runs
another instance of LSP Self-ping.
o Neither side forwards traffic through the LSP until local LSP
Self-ping returns TRUE.
The two LSP Self-ping sessions mentioned above are independent of one
another. They are not required to have the same Session-ID. Each
endpoint can forward traffic through the LSP as soon as its local LSP
Self-ping returns TRUE. Endpoints are not required to wait until
both LSP Self-ping sessions have returned TRUE.
6. IANA Considerations
IANA has assigned UDP Port Number 8503 [IANA.PORTS] for use by MPLS
LSP Self-Ping.
7. Security Considerations
LSP Self-ping messages are easily forged. Therefore, an attacker can
send the ingress LSR a forged LSP Self-ping message, causing the
ingress LSR to terminate the LSP Self-ping session prematurely. In
order to mitigate these threats, operators SHOULD filter LSP Self-
ping packets at the edges of the MPLS signaling domain. Furthermore,
implementations SHOULD NOT assign Session-IDs in a predictable
manner. In order to avoid predictability, implementations can
leverage a Cryptographically Secure Pseudorandom Number Generator
(CSPRNG) [NIST-CSPRNG].
8. References
8.1. Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <http://www.rfc-editor.org/info/rfc3704>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<http://www.rfc-editor.org/info/rfc6335>.
8.2. Informative References
[IANA.PORTS]
IANA, "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/
service-names-port-numbers>.
[NIST-CSPRNG]
NIST, "Recommendation for Random Number Generation Using
Deterministic Random Bit Generators", NIST Special
Publication 800-90A, January 2012.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<http://www.rfc-editor.org/info/rfc4594>.
[RFC6383] Shiomoto, K. and A. Farrel, "Advice on When It Is Safe to
Start Sending Data on Label Switched Paths Established
Using RSVP-TE", RFC 6383, DOI 10.17487/RFC6383, September
2011, <http://www.rfc-editor.org/info/rfc6383>.
[S-BFD] Akiya, N., Pignataro, C., Ward, D., Bhatia, M., and J.
Networks, "Seamless Bidirectional Forwarding Detection
(S-BFD)", Work in Progress, draft-ietf-bfd-seamless-
base-05, June 2015.
Appendix A. Rejected Approaches
In a rejected approach, the ingress LSR uses LSP Ping to verify LSP
readiness. This approach was rejected for the following reasons.
While an ingress LSR can control its control-plane overhead due to
LSP Ping, an egress LSR has no such control. This is because each
ingress LSR can, on its own, control the rate of the LSP Ping
originated by the LSR, while an egress LSR must respond to all the
LSP Pings originated by various ingresses. Furthermore, when an MPLS
Echo Request reaches an egress LSR, it is sent to the control plane
of the egress LSR; this makes egress LSR processing overhead of LSP
Ping well above the overhead of its data plane (MPLS/IP forwarding).
These factors make LSP Ping problematic as a tool for detecting LSP
readiness to carry traffic when dealing with a large number of LSPs.
By contrast, LSP Self-ping does not consume any control-plane
resources at the egress LSR, and it relies solely on the data plane
of the egress LSR, making it more suitable as a tool for checking LSP
readiness when dealing with a large number of LSPs.
In another rejected approach, the ingress LSR does not verify LSP
readiness. Instead, it sets a timer when it receives an RSVP RESV
message and does not forward traffic through the LSP until the timer
expires. This approach was rejected because it is impossible to
determine the optimal setting for this timer. If the timer value is
set too low, it does not prevent black-holing. If the timer value is
set too high, it slows down the process of LSP signaling and setup.
Moreover, the above-mentioned timer is configured on a per-router
basis. However, its optimum value is determined by a network-wide
behavior. Therefore, changes in the network could require changes to
the value of the timer, making the optimal setting of this timer a
moving target.
Acknowledgements
Thanks to Yakov Rekhter, Ravi Singh, Eric Rosen, Eric Osborne, Greg
Mirsky, and Nobo Akiya for their contributions to this document.
Contributors
The following individuals contributed significantly to this document:
Mark Wygant
Verizon
mark.wygant@verizon.com
Ravi Torvi
Juniper Networks
rtorvi@juniper.net
Authors' Addresses
Ron Bonica
Juniper Networks
Email: rbonica@juniper.net
Ina Minei
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States
Email: inaminei@google.com
Michael Conn
Verizon
Email: meconn26@gmail.com
Dante Pacella
Verizon
Email: dante.j.pacella@verizon.com
Luis Tomotaki
Verizon
Email: luis.tomotaki@verizon.com
AI ELI5
On this page
§1The Race Condition in RSVP-TE
Ingress Label Switching Routers (LSRs) use RSVP-TE (RFC 3209) to set up MPLS Label Switched Paths (LSPs). The ingress computes a path and sends a PATH message toward the egress along an Explicit Route Object. When the egress receives it, it binds a label, installs forwarding state if necessary, and sends a RESV message back upstream through the same routers in reverse. Each transit router binds a label and updates its forwarding state. When the ingress receives the RESV, it installs state and can forward traffic.
RFC 3209 says a node SHOULD be prepared to forward packets with the assigned label before sending the RESV, but does not strictly require it. Some implementations let routers send RESV before installing forwarding state (see RFC 6383), which lets routers install state in parallel and speeds up LSP setup. This creates a race condition: the ingress may start forwarding before downstream routers are ready, and traffic can be lost.
LSP Self-ping lets the ingress verify that forwarding state is installed on all downstream nodes. It is a new protocol, not an extension of LSP Ping (RFC 4379), and listens on its own UDP port. It is extremely lightweight and uses no control-plane resources on transit or egress LSRs.
Setting up a path is faster if every stop prepares at the same time. But then the first stop might say go before the last stop is ready, and things get lost.
§2When Self-ping Applies
LSP Self-ping fits when the ingress signals a point-to-point LSP, receives a RESV showing that downstream nodes have begun but not necessarily finished installing state, and needs to confirm they have finished. It also assumes that the ingress does not need to check that the state is correct (because that is very likely), that egress control-plane resources may be scarce, and that saving them matters more than checking correctness.
It is not a general-purpose MPLS OAM tool like LSP Ping or S-BFD. It cannot reliably tell whether state is correct. For example, it will not notice an LSP that terminates at the wrong node, or a router that wrongly forwards a packet without an MPLS label. It also fails when the LSP is signaled by LDP Independent Mode, or when Reverse Path Forwarding filters are enabled on links between ingress and egress.
LSP Ping uses control-plane resources on the egress, and S-BFD either does so or needs special forwarding-plane support. Self-ping needs nothing from the egress beyond the ability to forward an IP datagram. This matters during convergence events that affect a large number of LSPs.
The ball test only tells you the road is open, not that it leads to the right town. It is quick and bothers nobody at the far end.
§3The Self-ping Message
The message is a UDP packet that carries a session ID. If the RSVP messages that set up the LSP used IPv4, the UDP datagram MUST be in an IPv4 header; if they used IPv6, in an IPv6 header. In either case:
- The source address MAY be configurable and by default MUST be the address of the egress LSR.
- The destination address MUST be the address of the ingress LSR.
- TTL or hop count MAY be configurable and by default MUST be
255. - DSCP MAY be configurable and by default MUST be
CS6(110000). - The UDP source port MUST come from the dynamic range
49152-65535. - The UDP destination port MUST be
lsp-self-ping(8503).
The UDP payload is a single 64-bit Session-ID that associates a message with a Self-ping session.
The ball has your own home address and a number written on it, so when it rolls back you know which throw it was.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SESSION-ID | Session-ID | 8 bytes | Associates the message with a Self-ping session. |
§4Self-ping Procedures
The ingress creates a short-lived session and keeps all state locally: the 64-bit Session-ID, a Retry Counter (maximum number of probes, set by configuration), a Retry Timer (milliseconds to wait after each probe, set by configuration) and a boolean Status that starts FALSE.
Until Status is TRUE or Retry Counter reaches zero, the ingress formats a message with the Session-ID, sends it through the LSP under test, and starts the timer. If a message for the session returns, Status becomes TRUE. Otherwise the counter is decremented and the timer may optionally be increased by a back-off algorithm.
The message is addressed to the ingress itself. If forwarding state exists on all downstream LSRs, the egress sees it is addressed to the ingress and forwards it back like any other IP packet. It may reach the egress with or without an MPLS header, depending on penultimate hop popping, and the egress removes the header if present. If the egress's most preferred route back is through an LSP it uses that LSP; otherwise it sends the packet without MPLS encapsulation. When the session ends, the result is returned to the invoking protocol, such as RSVP-TE.
You keep tossing the ball, waiting a bit between tosses, until it comes back or you run out of tries.
- Self-ping (to ingress)Sent through the LSP under test.
- Self-ping
- Self-ping forwarded backForwarded like any other IP packet.
§5Bidirectional LSPs
A bidirectional LSP has an active side that calculates the ERO and signals the forward direction, and a passive side that reverses the ERO and signals the reverse direction. With Self-ping, each side runs its own session, and neither forwards traffic until its local Self-ping returns TRUE.
The two sessions are independent and need not use the same Session-ID. Each endpoint may start forwarding as soon as its own session succeeds, without waiting for the other.
Two friends each test their own direction of the road. Each can start walking as soon as their own ball returns.
§6Security and the Rejected Alternatives
Self-ping messages are easily forged, so an attacker could make the ingress end a session early. Operators SHOULD filter Self-ping packets at the edges of the MPLS signaling domain, and implementations SHOULD NOT assign Session-IDs predictably; a cryptographically secure pseudorandom number generator can help. IANA has assigned UDP port 8503 for MPLS LSP Self-Ping.
The appendix explains two rejected approaches. Using LSP Ping was rejected because each ingress controls its own probe rate but an egress must answer every ingress, and an MPLS Echo Request goes to the egress control plane, making the cost well above that of plain forwarding. Waiting on a fixed timer after RESV was rejected because no optimal value exists: too low does not prevent black-holing, too high slows LSP setup, and the best value depends on network-wide behavior, so it keeps changing.
Someone could throw a fake ball back to trick you, so use secret hard-to-guess numbers. Waiting a set time was dropped because nobody can pick the right time.