INTERNET-DRAFT Bidirectional Forwarding Detection A. Palanivelan Category: Standards Track Cisco Systems Expires: June 2010 December 1, 2009 Bidirectional Forwarding Detection (BFD) with Graceful Restart draft-palanivelan-bfd-v2-gr-03.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. 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A.Palanivelan [Page 1] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 Abstract This document proposes an extension for Bidirectional Forwarding Detection (BFD) to support Graceful restart, in complementing Graceful restart support of the underlying protocol.This shall work consistently irespective of the bfd mode or protocol or the type of restart.This document describes the challenges to bfd in surviving a graceful restart and a generic solution to succeed. Table of Contents 1 INTRODUCTION ............................................. 3 2 OVERVIEW ............................................... 3 3 MOTIVATIONS .............................................. 3 3.1 Planned and Unplanned Restarts with control protocols .. 4 3.2 BFD Co-existing with BB configs ....................... 4 4 Extensions to BFD .......................................... 5 4.1 Version (Vers)........................................ 5 4.2 Diagnostic (Diag)..................................... 6 4.3 My Restart Interval.................................... 6 4.4 Your Restart Interval.................................. 6 5 Theory of operation......................................... 7 6 Security Consderations...................................... 9 7 IANA Considerations......................................... 8 8 References ................................................. 9 8.1 Normative References................................... 9 8.2 Informative References................................. 9 9 Author's address............................................ 10 A.Palanivelan [Page 2] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 1. Introduction The Bidirectional Forwarding Detection protocol [BFD] provides a mechanism for liveness detection of arbitrary paths between systems. It is intended to provide low-overhead, short-duration detection of failures in the path between adjacent forwarding engines, including the interfaces, data link(s), and to the extent possible the forwarding engines themselves. It operates independently of media,data protocols,and routing protocols. An additional goal is to provide a single mechanism that can be used for liveness detection over any media, at any protocol layer, with a wide range of detection times and overhead, to avoid a proliferation of different methods. The extensions introduced in this draft for bfd shall aid in bfd complementing the GR capabilities of protocols such as ospf and also in providing a consistent behavior for planned/unplanned restarts irrespective of the underlying protocols. 2. Overview The Bidirectional Forwarding Detection [BFD] specification defines a protocol with simple and specific semantics. Its sole purpose is to verify connectivity between a pair of systems, for a particular data protocol across a path (which may be of any technology, length, or OSI layer). The promptness of the detection of a path failure can be controlled by trading off protocol overhead and system load with detection times. The extensions introduced in this draft for bfd shall aid in bfd complementing the GR capabilities of protocols such as ospf and also in providing a consistent behavior for planned/unplanned restarts for the underlying protocols. 3. Motivations Though the existing drafts discuss bfd interactions with applications with Graceful Restart and ways of implementing in serving successful GR, the drafts itself have some exceptions and caveats applied. This draft in particular discusses the issues in the following scenarios and provides a generic solution that would scale for future applications. A.Palanivelan [Page 3] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 * Unplanned restart. * Planned restart with a control protocol such as ISIS,which cannot signal GR. * BFD co-existing with BB configs This document tries to address the above issues in specific and Graceful restart mechanism in general, for bfd. 3.1 Planned and Unplanned Restarts with control protocols The existing bfd drafts suggest administratively disabling bfd prior to the start of GR. But, this works only for planned restarts and not for unplanned restarts. This also does not work for a protocol such as isis that cannot signal a planned restart. For a Planned restart where a control protocol can signal before restarting, if a BFD session failure occurs during the restart, it is recommended in the existing draft(s) that, such a planned restart SHOULD NOT be aborted and the session failure SHOULD NOT result in a topology change being signaled in the control protocol. Control protocols that cannot signal a planned restart depend on the recently restarted system to signal the Graceful Restart prior to the control protocol adjacency timeout. In most cases, whether the restart is planned or unplanned, it is likely that the BFD session will time out prior to the onset of Graceful Restart, and a topology change SHALL be signaled. This type of implementation shall impact non-stop routing and non-stop forwarding support using GR-enabled protocols and provides an opportunity to review the existing bfd implementations and improve. 3.2 BFD Co-existing with BB configs In a real time scenario with Broadband configurations,it is highly likely that the bfd sessions do not survive a Graceful restart. Assume a router at PE that has active DHCP sessions with a large number of clients (say 16k). During a planned restart, it is also likely that the DHCP clients request for renewal of IP address to the server (restarting router) at that time. When the router is restarting, these requests do not reach the router. But, when these requests reach the router when the router has just come up, it will treat these requests at a high priority and responds to them. When we have thousands of such requests to the restarting router, the router shall spend a major part of its first second of uptime in addressing these requests. In this scenario, a control protocol like ospfv2 that has GR enabled [OSPF-GRACE], shall withstand the restart for the specified restart interval (as it will be in seconds) and it is likely to survive the restart in maintaining its forwarding plane. In the same scenario, if bfd is enabled for ospfv2, for an unplanned A.Palanivelan [Page 4] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 restart, the (bfd) neighbor router will be expecting bfd control packets in milliseconds interval and during the restart process, is likely to timeout, also impacting the associated ospfv2 adjacency and resulting in loss of traffic. The scenario will be the same for bfd with a protocol such as is-is [ISIS-GRACE], where the problem is likely to be seen for a planned/unplanned restarts. 4. Extensions to BFD This draft introduces a new diag value to indicate that the neighbor is restarting and provisions to configure graceful restart timers. The Generic BFD Control Packet Format shown below introduces two additional sections "My Restart Interval" and "Your Restart Interval". 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Vers | Diag |Sta|P|F|C|A|D|M| Detect Mult | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | My Discriminator | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Your Discriminator | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Desired Min TX Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Required Min RX Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Required Min Echo RX Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | My Restart Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Your Restart Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.1 Version (Vers) The version of bfd defined by this draft, that has support for gr configuration and a diag for neighbor restarting state, shall have a value of 2. A.Palanivelan [Page 5] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 4.2 Diagnostic (Diag) A diagnostic code specifying the local system's reason for the last change in session state. A new diag value 9 for "Neighbor Restarting" is introduced in this draft. Values are: 0 -- No Diagnostic 1 -- Control Detection Time Expired 2 -- Echo Function Failed 3 -- Neighbor Signaled Session Down 4 -- Forwarding Plane Reset 5 -- Path Down 6 -- Concatenated Path Down 7 -- Administratively Down 8 -- Reverse Concatenated Path Down 9 - Neighbor Restarting 10-31 -- Reserved for future use This field allows remote systems to determine the reason that the previous session failed. 4.3 My Restart Interval This is the restart interval,in microseconds, of the transmitting system advertised to the remote system. In the case of a restart (of transmitting system), the remote system is xpected to keep the bfd session up for this duration of time. 4.4 Your Restart Interval The restart interval,in microseconds, received from the corresponding remote system. In the case of a restart (of remote system), the transmitting system is expected to keep the bfd session up for this duration of time. A.Palanivelan [Page 6] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 5. Theory of Operation The system that has support for high-availability, when using a routing protocol that is GR enabled, shall continue to forward traffic during a restart.when bfd is enabled on such a protocol, it is expected to assist the process than disturb it. With current bfd implementations, the bfd sessions do not survive a restart under different conditions.An Unplanned restart or a planned restart with a protocol such as isis that cannot signal about restart, are some of the conditions where bfd config is set to impact a high-availability situation. Though there are certain implementations adopted by various companies to make bfd survive restarts, there is no uniform method of achieving this and is likely to fail when interop with routers from other companies.This draft proposes a standard way of achieving this objective. This draft recommends the introduction of a new diag value (9 for "Neighbor restarting"), new version (2 for gr supported bfd) and two additional sections to the bfd packet format.This design is expected to provide a capability to bfd in withstanding restart scenarios, in complementing the associated protocol.This shall work consistently irespective of the bfd mode or protocol or the type of restart. This bfd implementation shall have its version field set to 2, that indicates the support for GR.The two new sections to the bfd packet format,"My Restart Interval" and "Your Restart interval" shall be used to exchange the GR timers info. between the systems. "My Restart Interval" is the time interval in microseconds,that this system expects its remote system to wait for, before bringing down its bfd session with this system. "Your Restart Interval" is the time interval in microseconds, specified by the remote system, that it expects this system, to wait for, before bringing down its bfd session with this system. The initial bfd packet exchange from the system to remote system shall have the configured value for the "My Restart Interval" or 0.The "Your Restart Interval" will reflect the value received in "My Restart Inteval" from the corresponding remote system or is Zero if value is unknown.A value of Zero for "Your Restart Interval" shall mean that the bfd gr is diabled at the remote end and similarly a value of Zero for "My Restart Interval" shall mean that bfd gr is disabled at the transmitting system.This effectively indicates to the other system if there is a requirement for it to wait for "restart interval" before timer expiry or not,but doesn't limit the system to act as a gr-helper.In other words, all systems with bfdv2 configurations shall act as GR helpers. Once the packet exchanges are complete and the bfd sessions are up,every bfd session will have info, about the time interval, its remote system will wait during a Restart and also the time interval this system has to wait,when the remote system restarts.The "My restart interval" value can be modified after the session is up,and A.Palanivelan [Page 7] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 in this case, the packet exchanges shall sync up the restart interval times (My and Your) on both the sides appropriately. The "My Restart Interval" configured shall have a value of 0 or a value more than its (Tx interval * Multiplier) value.The system shall not allow configuration of any value between these values. For Planned restarts, with bfd sessions succesfully up with a protocol such as ospfv2 and gr enabled,ospf shall signal its adjacent neighbor (al o a ospfv2-bfd neighbor) that a restart is about to begin. The bfd diag at the transmitting system (Restarter) shall be set to a value of 9 (Neighbor Restarting) if its "My Restart Interval" has a value other than 0, and bfd shall maintain its neighborship with the remote system for "Your Restart Interval" time(at remote neighbor) and the remote system shall not bring down the adjacency,knowing that the neighbor is restarting (known through the diag value). For Planned restarts, with bfd sessions succesfully up with a protocol such as isis and gr enabled,isis shall not signal its adjacent neighbor (also a isis-bfd neighbor) that a restart is about to begin.Here again, the remote system shall get to know that the other end is restarting,only after the restart begins.The bfd diag at the transmitting system (Restarter) shall be set to a value of 9 (Neighbor Restarting) if its "My Restart Interval" has a value other than 0, and bfd shall maintain its neighborship with the remote system for "Your Restart Interval" time (at remote neighbor) and the remote system shall not bring down the adjacency,knowing that the neighbor is restarting(known through the diag value). For Unplanned restarts,the system cannot signal to its neighbor that a restart is about to begin.In this scenario, Once the restart begins, the system will signal the restart,at the first possible instance.The bfd diag at the transmitting system(Restarter) shall be set to a value of 9 (Neighbor Restarting)if its "My Restart Interval" has a value other than 0, and bfd shall maintain its neighborship with the remote system for "Your Restart Interval" time (at remote neighbor) and the remote system shall not bring down the adjacency,knowing that the neighbor is restarting (known through the diag value). It may also be noted that attempting a GR for unplanned restart may not be a good idea, since the router may not properly prepare for a restart.The implementators, in this case,shall optionally provide a knob to turn the option off. A.Palanivelan [Page 8] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 6. Security Considerations Security considerations discussed in [BFD], [BFD-1HOP] and [BFD-MHOP] apply to this document. 7. IANA Considerations This document currently defines a Diag value of 9 to be used to specify "Neighbor Restarting" This document introduces two new sections "My Restart Interval" and "Your Restart Interval" to the bfd generic packet format. 8. References 8.1. Normative References [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", draft-ietf-bfd-base-09.txt, February, 2009. [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single Hop)", draft-ietf-bfd-v4v6-1hop-08.txt, February, 2009. [BFD-MULTI] Katz, D., and Ward, D., "BFD for Multihop Paths", draft- ietf-bfd-multihop-07.txt, February, 2009. 8.2. Informative References [ISIS-GRACE] Shand, M., and Ginsberg, L., "Restart signaling for IS- IS", RFC 5306, October 2008. [OSPF-GRACE] Moy, J., et al, "Graceful OSPF Restart", RFC 3623, November 2003. A.Palanivelan [Page 9] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009 Authors' Addresses Palanivelan A Cisco Systems, Bangalore,India. Email: apvelan@cisco.com A.Palanivelan [Page 10] Internet Draft draft-palanivelan-bfd-v2-gr-03.txt Dec 2009