Internet Draft                                              David Allan
 Document: draft-allan-mpls-loadbal-05.txt               Nortel Networks
                                                            October 2003
              A Suggested Approach for MPLS Load Spreading
 Status of this Memo
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 Copyright Notice
    Copyright(C) The Internet Society (2001). All Rights Reserved.
    RFC 3031 permits MPLS load spreading while making no specific
    representations as to implementation requirements. This has
    subsequently become an issue with respect to the reliability of path
    test mechanisms. Load spreading algorithms may separate path test
    probes from the path of interest. This document proposes suggestions
    for future implementations of load spreading such that path test
    mechanisms are not impacted.
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 Table of Contents
 1.    Introduction..................................................2
 2.    Discussion....................................................2
    2.1 Label Stack Entry Fields Modified by Intermediate LSRs.......4
    2.2 Reserved labels..............................................4
    2.3 Diffserv.....................................................5
    2.4 Monitored LSPs...............................................5
 3.    Guidelines....................................................5
 4.    Security Considerations.......................................7
 5.    References....................................................7
 6.    Acknowledgements..............................................7
 7.    Author's Address..............................................7
 1. Introduction
    MPLS load spreading mechanisms forward traffic with a common MPLS
    destination/forwarding equivalence class (FEC) over multiple
    parallel paths. The mechanisms to select which path are currently
    unspecified and frequently have payload and/or label stack
    dependencies. Protocols used for MPLS LSP verification not subject
    to load spreading will produce different results than payload types
    that are, and vice versa. Load spreading may involve examination of
    network layer addressing information. Even when a common network
    layer protocol is employed to transport both verification messages
    and traffic, a verification protocol's ability to impersonate
    traffic may be limited.
    This document proposes suggestions for future implementations of
    payload/label stack based load spreading such that path test
    mechanisms used for MPLS label switched path (LSP) verification and
    payload will have common forwarding behavior. This ensures that the
    LSP is properly verified by probe messages, and that the time to
    detect LSP problems is minimized.
    The term "level" is significant in this document. The definition is
    as defined in [RFC 3031]. This document makes the further
    distinction of "forwarding level" which is the level in the stack
    exclusive of reserved labels. So, for example, the presence of a
    router alert label on top of some arbitrary label stack does not
    alter the level relationship of non-reserved labels.
 2. Discussion
    The MPLS Architecture[RFC 3031] defines a number of data plane
    components internal to a label switch router (LSR) whose
    interactions define the topology of the components of a label
    switched path (LSP) local to that LSR. These are:
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    - The FEC to NHLFE (FTN) table which defines how incoming non-MPLS
      packets are classified and mapped to outgoing LSPs.
    - The Incoming Label Map (ILM) which defines how incoming LSPs are
      either terminated or cross connected to outgoing LSPs.
    - The Next Hop Label Forwarding Entry (NHLFE) which defines the
      steps to be performed to map incoming packets or LSPs to outgoing
      LSPs (outgoing interface and label stack operations).
    The MPLS Architecture[RFC 3031] and diffserv extensions [RFC3270]
    permit individual instances of the FTN and ILM tables to map to
    multiple NHLFEs, with the caveat that for a given packet, only one
    element from the set of NHLFEs must be selected for use before the
    packet is forwarded.
    What this means is that when the set of outgoing LSPs that an
    incoming packet may map to is more than one, how the LSR chooses the
    outgoing LSP has been left unspecified, and packets from a single
    source (for example an incoming LSP) may be spread out over more
    than one outgoing LSP. They will not get common treatment as the LSR
    attempts to spread the incoming load over the set of outgoing LSPs.
    When performed indiscriminately, this can result in misordering of
    packets and have other undesirable side effects.
    Therefore the load spreading algorithm, a.k.a. the NHLFE selection
    procedure, should have a number of desirable attributes:
         - Minimal re-ordering of packets in a flow. This is achieved by
         the selection mechanism ensuring packets in the same flow use
         the same NHLFE.
         - Path testing associated with a flow at any forwarding level
         use the same NHLFE as the flow itself. Otherwise, attempts to
         proactively detect or diagnose faults will produce inconsistent
         results. This is because the monitoring probes may use a
         different NHLFE than the flow.
         - Relative evenness in the distribution of traffic over the set
         of NHLFEs.
         - Preservation of diffserv characteristics.
         - Common forwarding between LSP associated functions and LSP
           payload above and beyond path testing mechanisms.
    One load spreading implementation selects the NHLFE based on a hash
    the label stack below the load shared level. It is assumed the depth
    of the stack is typically > 1, and the combination of stack depth,
    and the number of labels used at any given level will result in a
    reasonable distribution of traffic across the NHLFEs. Some
    implementations incorporate MPLS payload information into the NHLFE
    selection process as well.
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    As soon as any portion of the payload of an LSP is used as part of
    the NHLFE selection process, monitoring of that LSP will produce
    inconsistent results. This behavior is inherent to the load
    spreading process. The object of this draft is to provide guidelines
    such that operators may balance the need for testability and
    operational friendliness with the need for smooth randomization in
    load spreading.
 2.1 Label Stack Entry Fields Modified by Intermediate LSRs
    The ability for an LSP specific probe to follow the forwarding path
    of an LSP requires that some fields in the label stack must be
    ignored. The field value may vary from packet to packet. An example
    of this is time to live (TTL) when used in the uniform model [TTL].
    TTL reasonably could be expected to be consistent for an IP flow in
    a converged network (flow being expressed as some variation of a
    src/dst tuple), but an LSP may aggregate a number of flows, and may
    use probe packets with different TTL values than the payload. More
    importantly, incorporating TTL into NHLFE selection would play havoc
    with any TTL based traceroute mechanism.
 2.2 Reserved labels
    Reserved labels may be used to define LSP specific functions. A
    simplistic hash of the stack runs into problems if the hash of the
    label stack also includes reserved labels for MPLS functions that
    currently, or in the future, may also require common forwarding with
    the associated LSP. MPLS reserved functions associated with a
    specific LSP may resolve to a different NHLFE than the LSP payload.
    Reserved labels add to the stack depth (and are referred to as
    levels in that context), but carry functional rather than forwarding
    information. Examples would be the proposed OAM alert label
    [RFC3429], the Explicit V4 label or the router alert label. Other
    examples may emerge in the future.
    MPLS reserved labels are infrequently used. The inclusion of
    reserved label traffic for an LSP in the same NHLFE as the normal
    payload for that LSP should have negligible impact on the network
    engineering properties/evenness of distribution of traffic of a
    load-spread LSP.
    The set of reserved label values ranges from 0 to 15. A simple
    boolean 'and' of the label value with a mask should be sufficient to
    determine if information in that label stack entry should be
    included in the NHLFE selection process.
    The 'S' bit, indicating bottom of stack, does not uniquely identify
    the presence of forwarding information as it may indicate the
    presence of a reserved label. The 'S' bit should not be incorporated
    into the selection process.
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 2.3 Diffserv
    The existence of EXP-inferred LSPs (E-LSPs) means that a tested LSP
    may transport a number of diffserv class types. It would be
    desirable to be able to test/monitor only the LSP and not have to
    uniquely test/monitor each class type. To avoid inverse multiplexing
    of class types, EXP bits must be excluded from the selection
    process. Note that at a level ingress (either FTN or ILM) the EXP
    bits (or packet TOS bits) must be interpreted to ensure correct
    mapping of the diff-serv code point (DSCP as per [RFC3270]). They
    merely must be excluded from any simple randomization of packet
    forwarding across a multiplex group.
 2.4 Monitored LSPs
    An operator may want to monitor LSPs that transport MPLS LSPs. For
    example, a packet switched network (PSN) trunk for pseudo-wires
    (PWs). A simplistic hash of all the forwarding labels in the stack
    will mean that the monitored LSP and the payload of the monitored
    LSP may not have common forwarding. The hashing approach will only
    guarantee common forwarding for flows that have identical forwarding
    components in their label stacks. If the packet payload is also
    incorporated into the NHLFE selection process, the payload carrying
    LSP (bottom of the stack) may exhibit similar behavior.
    Accommodating this while providing for monitored LSPs is difficult,
    - Specific LSPs at arbitrary forwarding levels need to be able to be
    administratively designated as "monitored". This would be both
    unscalable and operationally intractable.
    - A range of label values is designated to specifically identify
    monitored LSPs (significant backwards compatibility issues)
    - The depth of the label stack (and payload) incorporated into the
    load spreading NHLFE selection process must be able to be
    administratively set. This trades off some evenness of distribution
    of traffic for testability and also means un-monitored LSPs will get
    the same treatment although they do not require it. Operationally
    this appears to be the most straightforward solution.
 3. Guidelines
    The following set of guidelines will permit load spreading to co-
    exist with path oriented verification tools that use reserved
    labels. It also permits IP tools to exercise load balacing
    constructs in a fixed amount of time.
    Although the discussion in this draft has focused on hashing-based
    NHLFE selection, the guidelines are sufficiently general to have
    broader applicability. These are:
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    1) The NHLFE selection procedures exclude the MPLS stack entries for
    any MPLS reserved labels [RFC 3032]. NHLFE selection procedures must
    resolve to the same NHLFE as they would if there were no reserved
    label(s) present.
    2) The NHLFE selection procedures for a stack that contains only
    reserved labels below the load-spread forwarding level always
    resolves to a common NHLFE.
    3) The NHLFE selection procedures exclude the 'S' bits from any
    label stack entries.
    4) The NHLFE selection procedures exclude the TTL field from any
    label stack entries.
    5) NHLFE selection procedures exclude the EXP bits for the labels
    incorporated into the selection process beyond ensuring that the
    selected NHLFE entry supports the outgoing PHB of the forwarded
    packet (FTN case) and the set of outgoing PHBs required by the ILM
    (ILM case).
    6) The depth of forwarding levels below the top label that is
    included in NHLFE selection procedures can be administratively
    configured. Levels with reserved labels do not contribute to depth
    establishment, nor are they included as per guideline 1 above.
    Implementations may include label stack forwarding information or
    packet payload in the selection process providing the depth does not
    exceed the administratively set boundary. If the level is
    administratively set to 'n', then forwarding labels at level 'n' or
    higher, or the packet payload of level 'n+1' or higher may be
    incorporated into the selection process.
    The scenarios supported by these guidelines are:
    1) When NHLFE selection input is administratively limited to the top
    of the stack or unlabelled packet, then testing/monitoring of all
    LSPs will produce consistent results. This will be true for both
    Y.1711 and LSP-PING (this eliminates the need to randomly manipulate
    the destination address to achieve fate sharing with the LSP under
    2) When NHLFE selection input is limited to the label stack, and the
    payload of an individual LSP is either another LSP or an unlabelled
    packet but not both, then testing/monitoring of all packet carrying
    LSPs (forwarding depth equals one) will produce consistent results.
    3) When NHLFE selection input is limited to the label stack, and the
    payload of an individual LSP can be another LSP or an unlabelled
    packet, then testing/monitoring of all LSPs at and below the
    administratively set level will produce consistent results.
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    4) When NHLFE selection input may include the label stack and
    payload then testing/monitoring of all LSPs at and below the
    administratively set level will produce consistent results.
 4. Security Considerations
    This draft introduces no new security issues into the MPLS
 5. References
    [RFC 3031] Rosen "Multiprotocol Label Switching
         Architecture", IETF RFC 3031, January 2001
    [RFC 3032] Rosen " MPLS Label Stack Encoding", IETF RFC
         3032, January 2001
    [RFC 3270] Le Faucheur, "MPLS Support of Differentiated
         Services", IETF RFC 3270, May 2002
    [RFC3429] Ohta, H., "Use of a reserved label value defined in RFC
         3032 for MPLS OAM functions", IETF RFC 3429, November
    [LSP-PING] Kompella "Detecting Data Plane Liveliness in
      MPLS", draft-ietf-mpls-lsp-ping-02 work in progress, March 2003
    [TTL] Agarwal " Time to Live (TTL) Processing in MPLS
         Networks (Updates RFC 3032)", IETF RFC 3443, February 2003
    [Y.1711] ITU-T Recommendation Y.1711, "OAM Mechanism for MPLS
         Networks", November 2002
 6. Acknowledgements
    Thanks to Shahram Davari and Neil Harrison for their detailed review
    of this draft.
 7. Author's Address
    David Allan
    Nortel Networks              Phone: 1-613-763-6362
    3500 Carling Ave.            Email:
    Ottawa, Ontario, CANADA