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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." Torvi, et al. Expires August 17, 2015 [Page 1] Internet-Draft LSP Self-Ping February 2015 This Internet-Draft will expire on August 17, 2015. Copyright Notice Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . 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. Torvi, et al. Expires August 17, 2015 [Page 2] Internet-Draft LSP Self-Ping February 2015 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. Torvi, et al. Expires August 17, 2015 [Page 3] Internet-Draft LSP Self-Ping February 2015 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, Torvi, et al. Expires August 17, 2015 [Page 4] Internet-Draft LSP Self-Ping February 2015 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- Torvi, et al. Expires August 17, 2015 [Page 5] Internet-Draft LSP Self-Ping February 2015 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. Torvi, et al. Expires August 17, 2015 [Page 6] Internet-Draft LSP Self-Ping February 2015 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. Torvi, et al. Expires August 17, 2015 [Page 7] Internet-Draft LSP Self-Ping February 2015 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. Torvi, et al. Expires August 17, 2015 [Page 8] Internet-Draft LSP Self-Ping February 2015 [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 Torvi, et al. Expires August 17, 2015 [Page 9] Internet-Draft LSP Self-Ping February 2015 Mark Wygant Verizon Email: mark.wygant@verizon.com Torvi, et al. Expires August 17, 2015 [Page 10]