MPLS Working Group R. Torvi
Internet-Draft R. Bonica
Intended status: Informational Juniper Networks
Expires: August 17, 2015 I. Minei
Google, Inc.
M. Conn
D. Pacella
L. Tomotaki
M. Wygant
Verizon
February 13, 2015
LSP Self-Ping
draft-bonica-mpls-self-ping-04
Abstract
LSP Self-ping is a new, light-weight protocol that ingress LSRs can
use to verify an LSPs readiness to carry traffic. LSP Self-ping does
not consume control plane resources on the egress LSR.
When an ingress LSR executes LSP Self-ping procedures, it constructs
a probe message. The probe message is an IP datagram whose
destination address represents an interface on the ingress LSR.
The ingress LSR forwards the probe through the LSP under test. If
the LSP is ready to forward traffic, the egress LSR receives the
probe. Because the probe is addressed to the ingress LSR, the egress
LSR forwards the probe back to the ingress. When the ingress LSR
receives the probe, it has verified LSP readiness without consuming
control plane resources at the egress LSR.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 17, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. LSP Self Ping Procedures . . . . . . . . . . . . . . . . . . 4
3. Bidirectional LSP Procedures . . . . . . . . . . . . . . . . 6
4. Rejected Approaches . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Ingress Label Switching Routers (LSR) can use RSVP-TE [RFC3209] to
establish a MPLS Label Switched Paths (LSP) [RFC3032]. The following
paragraphs outline RSVP-TE procedures.
The ingress LSR calculates 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) and the ERO
represents the calculated 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.
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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 and the Label Object contains 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 RESV message contains a Label Object and the Label
Object contains the label that has been bound to the LSP. 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.
Some implementations optimize the procedure described above by
allowing LSRs to send RESV messages before installing forwarding
state. 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 not have installed forwarding state yet. In this
case, if the ingress LSR forwards traffic through the LSP, traffic
will be black-holed until forwarding state is installed on all of the
downstream LSRs.
The ingress LSR can prevent back-holing by verifying the LSPs
readiness to carry traffic before forwarding traffic through it. LSP
Ping [RFC4379] and BFD [RFC5884] are mechanisms that the ingress LSR
could use to verify LSP readiness. However, LSP Ping and BFD consume
control plane resource on the egress LSR. During periods of network
restoration or reoptimzation, control plane resources may be scarce.
Therefore, a mechanism that does not consume control plane resources
on the egress LSR is required.
LSP Self-ping is a new, light-weight protocol that ingress LSRs can
use to verify an LSPs readiness to carry traffic. Unlike LSP Ping,
LSP Self-ping does not consume control plane resources on the egress
LSR.
When an ingress LSR executes LSP Self-ping procedures, it constructs
a probe message. The probe message is an IP datagram whose
destination address represents an interface on the ingress LSR.
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The ingress LSR forwards the probe through the LSP under test. If
the LSP is ready to forward traffic, the egress LSR receives the
probe. Because the probe is addressed to the ingress LSR, the egress
LSR forwards the probe back to the ingress. When the ingress LSR
receives the probe, it has verified LSP readiness without consuming
control plane resources at the egress LSR.
While LSP Self-ping does not consume control plane resources at the
egress LSR, it cannot detect some failures that can be detected by
protocols that consume control plane resources at the egress. For
example, LSP Self-ping cannot detect a misrouted LSP. Furthermore,
LSP Self-ping cannot be used to verify LSPs that were signaled using
LDP independent mode.
2. 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:
o Session-id: A 32-bit number that identifies the session
o verification-status: A boolean variable indicating whether LSP
readiness has been verified. The initial value of this variable
is FALSE.
o retries: The number of times that the ingress LSR probes the LSP
before giving up. 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.
The ingress LSR executes the following procedure until verification-
status equals TRUE or retries is less than 1:
o Format a MPLS Echo Reply [RFC4379] message
o Send the MPLS Echo Reply message through the LSP under test
o Set a timer to expire in retry-timer milliseconds
o Wait until either a) a MPLS Echo Reply message associated with the
session returns or b) the timer expires. If an MPLS Echo Reply
message associated with the session returns, set verification-
status to TRUE. Otherwise, decrement retries. Optionally,
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increase the value of retry-timer according to an appropriate back
off algorithm.
As per [RFC4379], the MPLS Echo Reply message is encapsulate in a
User Datagram Protocol (UDP) [RFC0768] header. If the protocol
messages used to establish the LSP were delivered over IPv4
[RFC0791], the UDP datagram is encapsulated in an IPv4 header. If
the protocol messages used to establish the LSP were delivered over
IPv6 [RFC2460], the UDP datagram is encapsulated in an IPv6 header.
In either case, message contents are as follows:
o IP Source Address is configurable. By default, it is the address
of the egress LSR
o IP Destination Address is the address of the ingress LSR
o IP Time to Live (TTL) / Hop Count is 255
o IP DSCP is configurable. By default, it is equal to CS6 (Ox48)
[RFC4594]
o UDP Source Port is any port selected from the dynamic range
(49152-65535) [RFC6335]
o UDP Destination Port is any port selected from the dynamic range
o MPLS Echo Global Flags are clear (i.e., set to 0)
o MPLS Echo Type is equal to "MPLS Echo Reply" (2)
o MPLS Echo Reply Mode is "Reply via an IPv4/IPv6 UDP packet" (2)
o MPLS Echo Senders Handle is equal to the Session-ID
o MPLS Echo Sequence Number is equal to retries
o MPLS Echo Time Stamp Sent is equal to the current time
The reader should note that the ingress LSR probes the LSP by sending
an MPLS Echo Reply message, addressed to itself, through the LSP.
The egress LSR forwards the MPLS Echo Reply message back to the
ingress LSR, exactly as it would forward any other IP packet.
If the LSP under test is ready to carry traffic, the egress LSR
receives the MPLS Echo Reply message. The MPLS Echo Reply message
can arrive at the egress LSR with or without an MPLS header,
depending on whether the LSP under test executes penultimate hop-
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popping procedures. If the MPLS Echo Reply message arrives at the
egress LSR with an MPLS header, the egress LSR removes that header.
The egress LSR forwards the MPLS Echo Reply message to its
destination, the ingress LSR. The egress LSR forwards the MPLS Echo
Reply message exactly as it would forward any other IP packet. If
the egress LSR's most preferred route to the ingress LSR is through
an LSP, the egress LSR forwards the MPLS Echo Reply 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 MPLS
Echo Reply message without MPLS encapsulation.
If the ingress LSR receives an MPLS Echo Reply message with Senders
Handle equal to the Session-ID, it sets the verification-status to
TRUE. The Sequence Number does not have to match the last Sequence
Number sent.
When an LSP Self-ping session terminates, it returns the value of
verification-status to the invoking protocol. For example, assume
that RSVP-TE invokes LSP Self-ping as part of the LSP set-up
procedure. If LSP Self-ping returns TRUE, RSVP-TE makes the LSP
under test available for forwarding. However, if LSP Self-ping
returns FALSE, RSVP-TE takes appropriate remedial actions.
LSP Self-ping fails if all of the following conditions are true:
o The Source Address of the MPLS Echo Reply message is equal to its
default value (that is, the address of the egress LSR)
o The penultimate hop pops the MPLS label
o The egress LSR executes Unicast Reverse Path Forwarding (uRPF)
procedures
In this scenario and in similar scenarios, the egress LSR discards
the MPLS Echo Reply message rather than forwarding it. In such
scenarios, the calling application can set the source address to a
more appropriate value.
3. Bidirectional LSP Procedures
In order to verify a bidirectional LSP's readiness to carry traffic,
the procedures described in Section 2 are executed twice, once by
each LSP endpoint. Each LSP endpoint tests LSP readiness in one
direction.
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4. Rejected Approaches
In a rejected approach, the ingress LSR uses LSP-Ping, exactly as
described in [RFC4379] to verify LSP readiness to carry traffic.
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, which 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 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. Alternatively, 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 signalling 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.
5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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6. Security Considerations
MPLS Echo messages are easily forged. Therefore, an attacker can
send the ingress LSR a forged MPLS Echo message, causing the ingress
LSR to terminate the LSP Self-ping session prematurely.
7. Acknowledgements
Thanks to Yakov Rekhter, Ravi Singh, Eric Rosen, Eric Osborne and
Nobo Akiya for their contributions to this document.
8. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594, August
2006.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
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[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, August 2011.
Authors' Addresses
Ravi Torvi
Juniper Networks
Email: rtorvi@juniper.net
Ron Bonica
Juniper Networks
Email: rbonica@juniper.net
Ina Minei
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
U.S.A.
Email: inaminei@google.com
Michael Conn
Verizon
Email: michael.e.conn@verizon.com
Dante Pacella
Verizon
Email: dante.j.pacella@verizon.com
Luis Tomotaki
Verizon
Email: luis.tomotaki@verizon.com
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Mark Wygant
Verizon
Email: mark.wygant@verizon.com
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