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Internet Engineering Task Force (IETF)                       K. ShiomotoRequest for Comments: 6383                                           NTTCategory: Informational                                        A. FarrelISSN: 2070-1721                                       Old Dog Consulting                                                          September 2011Advice on When It Is Safe to Start Sending Data onLabel Switched Paths Established Using RSVP-TEAbstract   The Resource Reservation Protocol (RSVP) has been extended to support   Traffic Engineering (TE) in Multiprotocol Label Switching (MPLS) and   Generalized MPLS (GMPLS) networks.  The protocol enables signaling   exchanges to establish Label Switched Paths (LSPs) that traverse   nodes and link to provide end-to-end data paths.  Each node is   programmed with "cross-connect" information as the signaling messages   are processed.  The cross-connection information instructs the node   how to forward data that it receives.   End points of an LSP need to know when it is safe to start sending   data so that it is not misdelivered, and so that safety issues   specific to optical data-plane technology are satisfied.  Likewise,   all label switching routers along the path of the LSP need to know   when to program their data planes relative to sending and receiving   control-plane messages.   This document clarifies and summarizes the RSVP-TE protocol exchanges   with relation to the programming of cross-connects along an LSP for   both unidirectional and bidirectional LSPs.  This document does not   define any new procedures or protocol extensions, and defers   completely to the documents that provide normative references.  The   clarifications set out in this document may also be used to help   interpret LSP establishment performance figures for MPLS-TE and GMPLS   devices.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   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).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.Shiomoto & Farrel             Informational                     [Page 1]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   Information about the current status of this document, any   errata, and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6383.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.1.  Introduction   The Resource Reservation Protocol (RSVP) [RFC2205] has been extended   to support Traffic Engineering (TE) in Multiprotocol Label Switching   (MPLS) and Generalized MPLS (GMPLS) networks [RFC3209] [RFC3473].   The protocol enables signaling exchanges to establish Label Switched   Paths (LSPs) that traverse nodes and links to provide end-to-end data   paths.  Each node is programmed with "cross-connect" information as   the signaling messages are processed.  The cross-connection   information instructs the node how to forward data that it receives.   In some technologies this requires configuration of physical devices,   while in others it may involve the exchange of commands between   different components of the node.  The nature of a cross-connect is   described further inSection 1.1.1.   End points of an LSP need to know when it is safe to start sending   data.  In this context "safe" has two meanings.  The first issue is   that the sender needs to know that the data path has been fully   established, setting up the cross-connects and removing any old,   incorrect forwarding instructions, so that data will be delivered to   the intended destination.  The other meaning of "safe" is that in   optical technologies, lasers must not be turned on until the correct   cross-connects have been put in place to ensure that service   personnel are not put at risk.   Similarly, all Label Switching Routers (LSRs) along the path of the   LSP need to know when to program their data planes relative to   sending and receiving control-plane messages.Shiomoto & Farrel             Informational                     [Page 2]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   This document clarifies and summarizes the RSVP-TE protocol exchanges   with relation to the programming of cross-connects along an LSP for   both unidirectional and bidirectional LSPs.  Bidirectional LSPs, it   should be noted, are supported only in GMPLS.  This document does not   define any new procedures or protocol extensions, and defers   completely to the documents that provide normative references.   The clarifications set out in this document may also be used to help   interpret LSP establishment performance figures for MPLS-TE and GMPLS   devices.  For example, the dynamic provisioning performance metrics   set out in [RFC5814] need to be understood in the context of LSP   setup times and not in terms of control message exchange times that   are actually only a component of the whole LSP establishment process.   Implementations could significantly benefit from this document   definitively identifying any LSR to forward the Path or Resv message   [RFC3473] before programming its cross-connect, thereby exploiting   pipelining (i.e., doing one action in the background while another is   progressing) to try to minimize the total time to set up the LSP.   However, while this document gives advice and identifies the issues   to be considered, it is not possible to make definitive statements   about how much pipelining is safe, since a node cannot "know" much   without first probing the network (for example, with protocol   extensions) which would defeat the point of pipelining.  Due to the   number of variables introduced by path length, and other node   behavior, ingress might be limited to a very pessimistic view for   safety.  Furthermore, it seems unlikely that an implementation would   necessarily give a full and frank description of how long it takes to   program and stabilize its cross-connects.  Nevertheless, this   document identifies the issues and opportunities for pipelining in   GMPLS systems.1.1.  Terminology   It is assumed that the reader is familiar with the basic message   flows of RSVP-TE as used in MPLS-TE and GMPLS.  Refer to [RFC2205],   [RFC3209], [RFC3471], and [RFC3473] for more details.1.1.1.  What is a Cross-Connect?   In the context of this document, the concept of a "cross-connection"   should be taken to imply the data forwarding instructions installed   (that is, "programmed") at a network node (or "switch").   In packet MPLS networks, this is often referred to as the Incoming   Label Map (ILM) and Next Hop Label Forwarding Entry (NHLFE) [RFC3031]   which are sometimes considered together as entries in the Label   Forwarding Information Base (LFIB) [RFC4221].  Where there isShiomoto & Farrel             Informational                     [Page 3]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   admission control and resource reservation associated with the data   forwarding path (such as the allocation of data buffers) [RFC3209],   this can be treated as part of the cross-connect programming process   since the LSP will not be available to forward data in the manner   agreed to during the signaling protocol exchange until the resources   are correctly allocated and reserved.   In non-packet networks (such as time-division multiplexing, or   optical switching networks), the cross-connect concept may be an   electronic cross-connect array or a transparent optical device (such   as a microelectromechanical system (MEMS)).  In all cases, however,   the concept applies to the instructions that are programmed into the   forwarding plane (that is, the data plane) so that incoming data for   the LSP on one port can be correctly handled and forwarded out of   another port.2.  Unidirectional MPLS-TE LSPs   [RFC3209] describes the RSVP-TE signaling and processing for MPLS-TE   packet-based networks.  LSPs in these networks are unidirectional by   definition (there are no bidirectional capabilities in [RFC3209]).Section 4.1.1.1 of [RFC3209] describes a node's process prior to   sending a Resv message to its upstream neighbor.      The node then sends the new LABEL object as part of the Resv      message to the previous hop.  The node SHOULD be prepared to      forward packets carrying the assigned label prior to sending the      Resv message.   This means that the cross-connect should be in place to support   traffic that may arrive at the node before the node sends the Resv.   This is clearly advisable because the upstream LSRs might otherwise   complete their cross-connections more rapidly and encourage the   ingress to start transmitting data with the risk that the node that   sent the Resv "early" would be unable to forward the data it received   and would be forced to drop it, or might accidentally send it along   the wrong LSP because of stale cross-connect information.   The use of "SHOULD" [RFC2119] in this text indicates that an   implementation could be constructed that sends a Resv before it is   ready to receive and forward data.  This might be done simply because   the internal construction of the node means that the control-plane   components cannot easily tell when the cross-connection has been   installed.  Alternatively, it might arise because the implementation   is aware that it will be slow and does not wish to hold up the   establishment of the LSP.  In this latter case, the implementation is   choosing to pipeline the cross-connect programming with the protocolShiomoto & Farrel             Informational                     [Page 4]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   exchange taking a gamble that there will be other upstream LSRs that   may also take some time to process, and it will in any case be some   time before the ingress actually starts to send data.  It should be   noted that, as well as the risks described in the previous paragraph,   a node that behaves like this must include a mechanism to report a   failure to chase the Resv message (using a PathErr) in the event that   the pipelined cross-connect processing fails.3.  GMPLS LSPs   GMPLS [RFC3945] extends RSVP-TE signaling for use in networks of   different technologies [RFC3471] [RFC3473].  This means that RSVP-TE   signaling may be used in MPLS packet switching networks, as well as   layer two networks (Ethernet, Frame Relay, ATM), time-division   multiplexing networks (Time Division Multiplexer (TDM), i.e.,   Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy   (SDH)), Wavelength Division Multiplexing (WDM) networks, and fiber   switched network.   The introduction of these other technologies, specifically the   optical technologies, brings about the second definition of the   "safe" commencement of data transmission as described inSection 1.   That is, there is a physical safety issue that means that the lasers   should not be enabled until the cross-connects are correctly in   place.   GMPLS supports unidirectional and bidirectional LSPs.  These are   split into separate sections for discussion.  The processing rules   are inherited from [RFC3209] unless they are specifically modified by   [RFC3471] and [RFC3473].3.1.  Unidirectional LSPs   Unidirectional LSP processing would be the same as that described inSection 2 except for the use of the Suggested_Label object defined in   [RFC3473].  This object allows an upstream LSR to 'suggest' to its   downstream neighbor the label that should be used for forward-   direction data by including the object on a Path message.  The   purpose of this object is to help the downstream LSR in its choice of   label, but it also makes it possible for the upstream LSR to   'pipeline' programming its cross-connect with the RSVP-TE signaling   exchanges.  That means that the cross-connect might be in place   before the signaling has completed (i.e., before a Resv message   carrying a Label object has been received at the upstream LSR).Shiomoto & Farrel             Informational                     [Page 5]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   We need to know when it is safe to start sending data.  There are   three sources of information.   -Section 3.4 of [RFC3471] states:      In particular, an ingress node should not transmit data traffic on      a suggested label until the downstream node passes a label      upstream.   The implication here is that an ingress node may (safely) start to   transmit data when it receives a label in a Resv message.   -Section 2.5 of [RFC3473] states:      Furthermore, an ingress node SHOULD NOT transmit data traffic      using a suggested label until the downstream node passes a      corresponding label upstream.   This is a confirmation of the first source.   -Section 4.1.1.1 of [RFC3209] states:      The node then sends the new LABEL object as part of the Resv      message to the previous hop.  The node SHOULD be prepared to      forward packets carrying the assigned label prior to sending the      Resv message.   In this text, the word "prior" is very important.  It means that the   cross-connect must be in place for forward traffic before the Resv is   sent.  In other words, each of the transit nodes and the egress node   must finish making their cross-connects before they send the Resv   message to their upstream neighbors.   Thus, as inSection 2, we can deduce that the ingress must not start   to transmit traffic until it has both received a Resv and has   programmed its own cross-connect.3.2.  Bidirectional LSPs   A bidirectional LSP is established with one signaling exchange of a   Path message from ingress to egress, and a Resv from egress to   ingress.  The LSP itself is comprised of two sets of forwarding   state, one providing a path from the ingress to the egress (the   forwards data path), and one from the egress to the ingress (the   reverse data path).Shiomoto & Farrel             Informational                     [Page 6]

RFC 6383            RVSP-TE Data Label Switch Update      September 20113.2.1.  Forwards Direction Data   The processing for the forwards direction data path is exactly as   described for a unidirectional LSP inSection 3.1.3.2.2.  Reverse Direction Data   For the reverse direction data flow, an Upstream_Label object is   carried in the Path message from each LSR to its downstream neighbor.   The Upstream_Label object tells the downstream LSR which label to use   for data being sent to the upstream LSR (that is, reverse direction   data).  The use of the label is confirmed by the downstream LSR when   it sends a Resv message.  Note that there is no explicit confirmation   of the label in the Resv message, but if the label was not acceptable   to the downstream LSR, it would return a PathErr message instead.   The upstream LSR must decide when to send the Path message relative   to when it programs its cross-connect.  That is:   -  Should it program the cross-connect before it sends the Path      message;   -  Can it overlap the programming with the exchange of messages; or   -  Must it wait until it receives a Resv from its downstream      neighbor?   The defining reference isSection 3.1 of [RFC3473]:      The Upstream_Label object MUST indicate a label that is valid for      forwarding at the time the Path message is sent.   In this text, "valid for forwarding" should be taken to mean that it   is safe for the LSR that sends the Path message to receive data, and   that the LSR will forward data correctly.  The text does not mean   that the label is "acceptable for use" (i.e., the label is available   to be cross-connected).   This point is clarified later inSection 3.1 of [RFC3473]:      Terminator nodes process Path messages as usual, with the      exception that the upstream label can immediately be used to      transport data traffic associated with the LSP upstream towards      the initiator.   This is a clear statement that when a Path message has been fully   processed by an egress node, it is completely safe to transmit data   toward the ingress (i.e., reverse direction data).Shiomoto & Farrel             Informational                     [Page 7]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   From this we can deduce several things:   -  An LSR must not wait to receive a Resv message before it programs      the cross-connect for the reverse direction data.  It must be      ready to receive data from the moment that the egress completes      processing the Path message that it receives (i.e., before it      sends a Resv back upstream).   -  An LSR may expect to start receiving reverse direction data as      soon as it sends a Path message for a bidirectional LSP.   -  An LSR may make some assumptions about the time lag between      sending a Path message and the message reaching and being      processed by the egress.  It may take advantage of this time lag      to pipeline programming the cross-connect.3.3.  ResvConf Message   The ResvConf message is used in standard RSVP [RFC2205] to let the   ingress confirm to the egress that the Resv has been successfully   received, and what bandwidth has been reserved.  In RSVP-TE [RFC3209]   and GMPLS [RFC3473], it is not expected that bandwidth will be   modified along the path of the LSP, so the purpose of the ResvConf is   reduced to a confirmation that the LSP has been successfully   established.   The egress may request that a ResvConf be sent by including a   Resv_Confirm object in the Resv message that it sends.  When the   ingress receives the Resv message and sees the Resv_Confirm object,   it can respond with a ResvConf message.   It should be clear that this mechanism might provide a doubly secure   way for the egress to ensure that the reverse direction data path is   safely in place before transmitting data.  That is, if the egress   waits until it receives a ResvConf message, it can be sure that the   whole LSP is in place.   However, this mechanism is excessive given the definitions presented   inSection 3.2.2, and would delay LSP setup by one end-to-end message   propagation cycle.  It should be noted as well that the generation   and of the ResvConf message is not guaranteed.  Furthermore, many (if   not most) GMPLS implementations neither request nor send ResvConf   messages.  Therefore, egress reliance  on the receipt of a ResvConf   as a way of knowing that it is safe to start transmitting reverse   direction data is not recommended.Shiomoto & Farrel             Informational                     [Page 8]

RFC 6383            RVSP-TE Data Label Switch Update      September 20113.4.  Administrative Status   GMPLS offers an additional tool for ensuring safety of the LSP.  The   Administrative Status information is defined inSection 8 of   [RFC3471] and is carried in the Admin_Status Object defined inSection 7 of [RFC3473].   This object allows an ingress to set up an LSP in "Administratively   Down" state.  This state means that [RFC3471]:      ... the local actions related to the "administratively down" state      should be taken.   In this state, it is assumed that the LSP exists (i.e., the cross-   connects are all in place), but no data is transmitted (i.e., in   optical systems, the lasers are off).   Additionally, the Admin_Status object allows the LSP to be put into   "Testing" state.  This state means ([RFC3471]) that:      ... the local actions related to the "testing" mode should be      taken.   This state allows the connectivity of the LSP to be tested without   actually exchanging user data.  For example, in an optical system, it   would be possible to run a data continuity test (using some external   coordination of errors).  In a packet network, a connection   verification exchange (such as the in-band Virtual Circuit   Connectivity Verification described inSection 5.1.1 of [RFC5085])   could be used.  Once connectivity has been verified, the LSP could be   put into active mode and the exchange of user data could commence.   These processes may be considered particularly important in systems   where the control-plane processors are physically distinct from the   data-plane cross-connects (for example, where there is a   communication protocol operating between the control-plane processor   and the data-plane switch) in which case the successful completion of   control-plane signaling cannot necessarily be taken as evidence of   correct data-plane programming.4.  Implications for Performance Metrics   The ability of LSRs to handle and propagate control-plane messages   and to program cross-connects varies considerably from device to   device according to switching technology, control-plane connectivity,   and implementation.  These factors influence how quickly an LSP can   be established.Shiomoto & Farrel             Informational                     [Page 9]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   Different applications have different requirements for the speed of   setup of LSPs, and this may be particularly important in recovery   scenarios.  It is important for service providers considering the   deployment of MPLS-TE or GMPLS equipment to have a good benchmark for   the performance of the equipment.  Similarly, it is important for   equipment vendors to be compared on a level playing field.   In order to provide a basis for comparison, [RFC5814] defines a   series of performance metrics to evaluate dynamic LSP provisioning   performance in MPLS-TE/GMPLS networks.  Any use of such metrics must   be careful to understand what is being measured, bearing in mind that   it is not enough to know that the control-plane message has been   processed and forwarded: the cross-connect must be put in place   before the LSP can be used.  Thus, care must be taken to ensure that   devices are correctly conforming to the procedures clarified inSection 2 of this document, and not simply forwarding control-plane   messages with the intent to program the cross-connects in the   background.5.  Security Considerations   This document does not define any network behavior and does not   introduce or seek to solve any security issues.   It may be noted that a clear understanding of when to start sending   data may reduce the risk of data being accidentally delivered to the   wrong place or individuals being hurt.6.  Acknowledgments   Thanks to Weiqiang Sun, Olufemi Komolafe, Daniel King, and Stewart   Bryant for their review and comments.7.  References7.1.  Normative References   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.             Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1             Functional Specification",RFC 2205, September 1997.   [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.Shiomoto & Farrel             Informational                    [Page 10]

RFC 6383            RVSP-TE Data Label Switch Update      September 2011   [RFC3471] Berger, L., Ed., "Generalized Multi-Protocol Label             Switching (GMPLS) Signaling Functional Description",RFC3471, January 2003.   [RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label             Switching (GMPLS) Signaling Resource ReserVation Protocol-             Traffic Engineering (RSVP-TE) Extensions",RFC 3473,             January 2003.   [RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label             Switching (GMPLS) Architecture",RFC 3945, October 2004.7.2.  Informative References   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol             Label Switching Architecture",RFC 3031, January 2001.   [RFC4221] Nadeau, T., Srinivasan, C., and A. Farrel, "Multiprotocol             Label Switching (MPLS) Management Overview",RFC 4221,             November 2005.   [RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual             Circuit Connectivity Verification (VCCV): A Control Channel             for Pseudowires",RFC 5085, December 2007.   [RFC5814] Sun, W., Ed., and G. Zhang, Ed., "Label Switched Path (LSP)             Dynamic Provisioning Performance Metrics in Generalized             MPLS Networks",RFC 5814, March 2010.Authors' Addresses   Kohei Shiomoto   NTT Service Integration Laboratories   3-9-11 Midori   Musashino, Tokyo 180-8585   Japan   Phone: +81 422 59 4402   EMail: shiomoto.kohei@lab.ntt.co.jp   Adrian Farrel   Old Dog Consulting   EMail: adrian@olddog.co.ukShiomoto & Farrel             Informational                    [Page 11]