IS-IS Fast Flooding
draft-ietf-lsr-isis-fast-flooding-11

Network Working Group                                        B. Decraene
Internet-Draft                                                    Orange
Intended status: Experimental                                L. Ginsberg
Expires: 14 November 2024                                  Cisco Systems
                                                                   T. Li
                                                  Juniper Networks, Inc.
                                                             G. Solignac
                                                                        
                                                              M. Karasek
                                                           Cisco Systems
                                                         G. Van de Velde
                                                                   Nokia
                                                           T. Przygienda
                                                                 Juniper
                                                             13 May 2024

                          IS-IS Fast Flooding
                  draft-ietf-lsr-isis-fast-flooding-11

Abstract

   Current Link State Protocol Data Unit (PDU) flooding rates are much
   slower than what modern networks can support.  The use of IS-IS at
   larger scale requires faster flooding rates to achieve desired
   convergence goals.  This document discusses the need for faster
   flooding, the issues around faster flooding, and some example
   approaches to achieve faster flooding.  It also defines protocol
   extensions relevant to faster flooding.

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
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   This Internet-Draft will expire on 14 November 2024.

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Copyright Notice

   Copyright (c) 2024 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Historical Behavior . . . . . . . . . . . . . . . . . . . . .   4
   4.  Flooding Parameters TLV . . . . . . . . . . . . . . . . . . .   5
     4.1.  LSP Burst Size sub-TLV  . . . . . . . . . . . . . . . . .   6
     4.2.  LSP Transmission Interval sub-TLV . . . . . . . . . . . .   6
     4.3.  LSPs Per PSNP sub-TLV . . . . . . . . . . . . . . . . . .   6
     4.4.  Flags sub-TLV . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Partial SNP Interval sub-TLV  . . . . . . . . . . . . . .   7
     4.6.  Receive Window sub-TLV  . . . . . . . . . . . . . . . . .   8
     4.7.  Operation on a LAN interface  . . . . . . . . . . . . . .   8
   5.  Performance improvement on the receiver . . . . . . . . . . .   9
     5.1.  Rate of LSP Acknowledgments . . . . . . . . . . . . . . .   9
     5.2.  Packet Prioritization on Receive  . . . . . . . . . . . .  10
   6.  Congestion and Flow Control . . . . . . . . . . . . . . . . .  11
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Congestion and Flow Control algorithm . . . . . . . . . .  11
     6.3.  Transmitter Based Congestion Control Approach . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  Flooding Parameters TLV . . . . . . . . . . . . . . . . .  21
     7.2.  Registry: IS-IS Sub-TLV for Flooding Parameters TLV . . .  21
     7.3.  Registry: IS-IS Bit Values for Flooding Parameters Flags
           Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  24
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Changes / Author Notes . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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1.  Introduction

   Link state IGPs such as Intermediate-System-to-Intermediate-System
   (IS-IS) depend upon having consistent Link State Databases (LSDB) on
   all Intermediate Systems (ISs) in the network in order to provide
   correct forwarding of data packets.  When topology changes occur,
   new/updated Link State PDUs (LSPs) are propagated network-wide.  The
   speed of propagation is a key contributor to convergence time.

   IS-IS base specification [ISO10589] does not use flow or congestion
   control but static flooding rates.  Historically, flooding rates have
   been conservative - on the order of 10s of LSPs/second.  This is the
   result of guidance in the base specification and early deployments
   when the CPU and interface speeds were much slower and the area scale
   much smaller than they are today.

   As IS-IS is deployed in greater scale both in the number of nodes in
   an area and in the number of neighbors per node, the impact of the
   historic flooding rates becomes more significant.  Consider the
   bringup or failure of a node with 1000 neighbors.  This will result
   in a minimum of 1000 LSP updates.  At typical LSP flooding rates used
   today (33 LSPs/second), it would take more than 30 seconds simply to
   send the updated LSPs to a given neighbor.  Depending on the diameter
   of the network, achieving a consistent LSDB on all nodes in the
   network could easily take a minute or more.

   Increasing the LSP flooding rate therefore becomes an essential
   element of supporting greater network scale.

   Improving the LSP flooding rate is complementary to protocol
   extensions that reduce LSP flooding traffic by reducing the flooding
   topology such as Mesh Groups [RFC2973] or Dynamic Flooding
   [I-D.ietf-lsr-dynamic-flooding] . Reduction of the flooding topology
   does not alter the number of LSPs required to be exchanged between
   two nodes, so increasing the overall flooding speed is still
   beneficial when such extensions are in use.  It is also possible that
   the flooding topology can be reduced in ways that prefer the use of
   neighbors that support improved flooding performance.

   With the goal of supporting faster flooding, this document introduces
   the signaling of additional flooding related parameters Section 4,
   specifies some performance improvements on the receiver Section 5 and
   introduces the use of flow and/or congestion control Section 6.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Historical Behavior

   The base specification for IS-IS [ISO10589] was first published in
   1992 and updated in 2002.  The update made no changes in regards to
   suggested timer values.  Convergence targets at the time were on the
   order of seconds and the specified timer values reflect that.  Here
   are some examples:

   minimumLSPGenerationInterval - This is the minimum time interval
        between generation of Link State PDUs. A source Intermediate
        system shall wait at least this long before re-generating one
        of its own Link State PDUs.

   The recommended value is 30 seconds.

   minimumLSPTransmissionInterval - This is the amount of time an
        Intermediate system shall wait before further propagating
        another Link State PDU from the same source system.

   The recommended value is 5 seconds.

   partialSNPInterval - This is the amount of time between periodic
        action for transmission of Partial Sequence Number PDUs.
        It shall be less than minimumLSPTransmissionInterval.

   The recommended value is 2 seconds.

   Most relevant to a discussion of the LSP flooding rate is the
   recommended interval between the transmission of two different LSPs
   on a given interface.

   For broadcast interfaces, [ISO10589] defined:

     minimumBroadcastLSPTransmissionInterval - the minimum interval
        between PDU arrivals which can be processed by the slowest
        Intermediate System on the LAN.

   The default value was defined as 33 milliseconds.  It is permitted to
   send multiple LSPs "back-to-back" as a burst, but this was limited to
   10 LSPs in a one second period.

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   Although this value was specific to LAN interfaces, this has commonly
   been applied by implementations to all interfaces though that was not
   the original intent of the base specification.  In fact
   Section 12.1.2.4.3 states:

     On point-to-point links the peak rate of arrival is limited only
     by the speed of the data link and the other traffic flowing on
     that link.

   Although modern implementations have not strictly adhered to the 33
   millisecond interval, it is commonplace for implementations to limit
   the flooding rate to the same order of magnitude: tens of
   milliseconds, and not the single digits or fractions of milliseconds
   that are needed today.

   In the past 20 years, significant work on achieving faster
   convergence, more specifically sub-second convergence, has resulted
   in implementations modifying a number of the above timers in order to
   support faster signaling of topology changes.  For example,
   minimumLSPGenerationInterval has been modified to support millisecond
   intervals, often with a backoff algorithm applied to prevent LSP
   generation storms in the event of rapid successive oscillations.

   However, the flooding rate has not been fundamentally altered.

4.  Flooding Parameters TLV

   This document defines a new Type-Length-Value tuple (TLV) called the
   "Flooding Parameters TLV" that may be included in IS to IS Hellos
   (IIH) or Partial Sequence Number PDUs (PSNPs).  It allows IS-IS
   implementations to advertise flooding-related parameters and
   capabilities which may be used by the peer to support faster
   flooding.

   Type: 21

   Length: variable, the size in octets of the Value field

   Value: One or more sub-TLVs

   Several sub-TLVs are defined in this document.  The support of any
   sub-TLV is OPTIONAL.

   For a given IS-IS adjacency, the Flooding Parameters TLV does not
   need to be advertised in each IIH or PSNP.  An IS uses the latest
   received value for each parameter until a new value is advertised by
   the peer.  However, as IIHs and PSNPs are not reliably exchanged, and
   may never be received, parameters SHOULD be sent even if there is no

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   change in value since the last transmission.  For a parameter that
   has never been advertised, an IS uses its local default value.  That
   value SHOULD be configurable on a per-node basis and MAY be
   configurable on a per-interface basis.

4.1.  LSP Burst Size sub-TLV

   The LSP Burst Size sub-TLV advertises the maximum number of LSPs that
   the node can receive without an intervening delay between LSP
   transmissions.

   Type: 1

   Length: 4 octets

   Value: number of LSPs that can be received back-to-back.

4.2.  LSP Transmission Interval sub-TLV

   The LSP Transmission Interval sub-TLV advertises the minimum
   interval, in micro-seconds, between LSPs arrivals which can be
   sustained on this receiving interface.

   Type: 2

   Length: 4 octets

   Value: minimum interval, in micro-seconds, between two consecutive
   LSPs received after LSP Burst Size LSPs have been received

   The LSP Transmission Interval is an advertisement of the receiver's
   sustainable LSP reception rate.  This rate may be safely used by a
   sender which do not support the flow control or congestion algorithm.
   It may also be used as the minimal safe rate by flow control or
   congestion algorithms in unexpected cases, e.g., when the receiver is
   not acknowledging LSPs anymore.

4.3.  LSPs Per PSNP sub-TLV

   The LSP per PSNP (LPP) sub-TLV advertises the number of received LSPs
   that triggers the immediate sending of a PSNP to acknowledge them.

   Type: 3

   Length: 2 octets

   Value: number of LSPs acknowledged per PSNP

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   A node advertising this sub-TLV with a value for LPP MUST send a PSNP
   once LPP LSPs have been received and need to be acknowledged.

4.4.  Flags sub-TLV

   The sub-TLV Flags advertises a set of flags.

   Type: 4

   Length: Indicates the length in octets (1-8) of the Value field.  The
   length SHOULD be the minimum required to send all bits that are set.

   Value: List of flags.

             0 1 2 3 4 5 6 7 ...
            +-+-+-+-+-+-+-+-+...
            |O|              ...
            +-+-+-+-+-+-+-+-+...

   An LSP receiver sets the O-flag to indicate to the LSP sender that it
   will acknowledge the LSPs in the order as received.  A PSNP
   acknowledging N LSPs is acknowledging the N oldest LSPs received.
   The order inside the PSNP is meaningless.  If the sender keeps track
   of the order of LSPs sent, this indication allows a fast detection of
   the loss of an LSP.  This MUST NOT be used to alter the
   retransmission timer for any LSP.  This MAY be used to trigger a
   congestion signal.

4.5.  Partial SNP Interval sub-TLV

   The Partial SNP Interval sub-TLV advertises the amount of time in
   milliseconds between periodic action for transmission of Partial
   Sequence Number PDUs.  This time will trigger the sending of a PSNP
   even if the number of unacknowledged LSPs received on a given
   interface does not exceed LPP (Section 4.3).  The time is measured
   from the reception of the first unacknowledged LSP.

   Type: 5

   Length: 2 octets

   Value: partialSNPInterval in milliseconds

   A node advertising this sub-TLV SHOULD send a PSNP at least once per
   Partial SNP Interval if one or more unacknowledged LSPs have been
   received on a given interface.

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4.6.  Receive Window sub-TLV

   The Receive Window (RWIN) sub-TLV advertises the maximum number of
   unacknowledged LSPs that the node can receive for a given adjacency.

   Type: 6

   Length: 2 octets

   Value: maximum number of unacknowledged LSPs

4.7.  Operation on a LAN interface

   On a LAN interface, all LSPs are link-level multicasts.  Each LSP
   sent will be received by all ISs on the LAN and each IS will receive
   LSPs from all transmitters.  In this section, we clarify how the
   flooding parameters should be interpreted in the context of a LAN.

   An LSP receiver on a LAN will communicate its desired flooding
   parameters using a single Flooding Parameters TLV, which will be
   received by all LSP transmitters.  The flooding parameters sent by
   the LSP receiver MUST be understood as instructions from the LSP
   receiver to each LSP transmitter about the desired maximum transmit
   characteristics of each transmitter.  The receiver is aware that
   there are multiple transmitters that can send LSPs to the receiver
   LAN interface.  The receiver might want to take that into account by
   advertising more conservative values, e.g., a higher LSP Transmission
   Interval.  When the transmitters receive the LSP Transmission
   Interval value advertised by an LSP receiver, the transmitters should
   rate-limit LSPs according to the advertised flooding parameters.
   They should not apply any further interpretation to the flooding
   parameters advertised by the receiver.

   A given LSP transmitter will receive multiple flooding parameter
   advertisements from different receivers that may include different
   flooding parameter values.  A given transmitter SHOULD use the most
   convervative value on a per-parameter basis.  For example, if the
   transmitter receives multiple LSP Burst Size values, it should use
   the smallest value.

   The Designated Intermediate System (DIS) plays a special role in the
   operation of flooding on the LAN as it is responsible for responding
   to PSNPs sent on the LAN circuit which are used to request LSPs that
   the sender of the PSNP does not have.  If the DIS does not support
   faster flooding, this will impact the maximum flooding speed which
   could occur on a LAN.  Use of LAN priority to prefer a node which
   supports faster flooding in the DIS election may be useful.

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   NOTE: The focus of work used to develop the example algorithms
   discussed later in this document focused on operation over point-to-
   point interfaces.  A full discussion of how best to do faster
   flooding on a LAN interface is therefore out of scope for this
   document.

5.  Performance improvement on the receiver

   This section defines two behaviors that SHOULD be implemented on the
   receiver.

5.1.  Rate of LSP Acknowledgments

   On point-to-point networks, PSNPs provide acknowledgments for
   received LSPs.  [ISO10589] suggests that some delay be used when
   sending PSNPs.  This provides some optimization as multiple LSPs can
   be acknowledged by a single PSNP.

   Faster LSP flooding benefits from a faster feedback loop.  This
   requires a reduction in the delay in sending PSNPs.

   For the generation of PSNPs, the receiver SHOULD use a
   partialSNPInterval smaller than the one defined in [ISO10589].  The
   choice of this lower value is a local choice.  It may depend on the
   available processing power of the node, the number of adjacencies,
   and the requirement to synchronize the LSDB more quickly. 200 ms
   seems to be a reasonable value.

   In addition to the timer-based partialSNPInterval, the receiver
   SHOULD keep track of the number of unacknowledged LSPs per circuit
   and level.  When this number exceeds a preset threshold of LSPs Per
   PSNP (LPP), the receiver SHOULD immediately send a PSNP without
   waiting for the PSNP timer to expire.  In the case of a burst of
   LSPs, this allows for more frequent PSNPs, giving faster feedback to
   the sender.  Outside of the burst case, the usual time-based PSNP
   approach comes into effect.

   The smaller the LPP, the faster the feedback to the sender and
   possibly the higher the rate if the rate is limited by the end to end
   RTT (link RTT + time to acknowledge).  This may result in an increase
   in the number of PSNPs sent which may increase CPU and IO load on
   both the sender and receiver.  The LPP should be less than or equal
   to 90 as this is the maximum number of LSPs that can be acknowledged
   in a PSNP at common MTU sizes, hence waiting longer would not reduce
   the number of PSNPs sent but would delay the acknowledgements.  LPP
   should not be chosen too high as the congestion control starts with a
   congestion window of LPP+1.  Based on experimental evidence, 15
   unacknowledged LSPs is a good value assuming that the Receive Window

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   is at least 30.  More frequent PSNPs gives the transmitter more
   feedback on receiver progress, allowing the transmitter to continue
   transmitting while not burdening the receiver with undue overhead.

   By deploying both the time-based and the threshold-based PSNP
   approaches, the receiver can be adaptive to both LSP bursts and
   infrequent LSP updates.

   As PSNPs also consume link bandwidth, packet-queue space, and
   protocol-processing time on receipt, the increased sending of PSNPs
   should be taken into account when considering the rate at which LSPs
   can be sent on an interface.

5.2.  Packet Prioritization on Receive

   There are three classes of PDUs sent by IS-IS:

   *  Hellos

   *  LSPs

   *  Complete Sequence Number PDUs (CSNPs) and PSNPs

   Implementations today may prioritize the reception of Hellos over
   LSPs and Sequence Number PDUs (SNPs) in order to prevent a burst of
   LSP updates from triggering an adjacency timeout which in turn would
   require additional LSPs to be updated.

   CSNPs and PSNPs serve to trigger or acknowledge the transmission of
   specified LSPs.  On a point-to-point link, PSNPs acknowledge the
   receipt of one or more LSPs.  For this reason, [ISO10589] specifies a
   delay (partialSNPInterval) before sending a PSNP so that the number
   of PSNPs required to be sent is reduced.  On receipt of a PSNP, the
   set of LSPs acknowledged by that PSNP can be marked so that they do
   not need to be retransmitted.

   If a PSNP is dropped on reception, the set of LSPs advertised in the
   PSNP cannot be marked as acknowledged and this results in needless
   retransmissions that will further delay transmission of other LSPs
   that are yet to be transmitted.  It may also make it more likely that
   a receiver becomes overwhelmed by LSP transmissions.

   Therefore implementations SHOULD prioritize IS-IS PDUs on the way
   from the incoming interface to the IS-IS process.  The relative
   priority of packets in decreasing order SHOULD be: Hellos, SNPs,
   LSPs.  Implementations MAY also prioritize IS-IS packets over other
   protocols which are less critical for the router or network, less
   sensitive to delay or more bursty (e.g., BGP).

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6.  Congestion and Flow Control

6.1.  Overview

   Ensuring the goodput between two entities is a layer-4 responsibility
   as per the OSI model.  A typical example is the TCP protocol defined
   in [RFC9293] that provides flow control, congestion control, and
   reliability.

   Flow control creates a control loop between a transmitter and a
   receiver so that the transmitter does not overwhelm the receiver.
   TCP provides a means for the receiver to govern the amount of data
   sent by the sender through the use of a sliding window.

   Congestion control prevents the set of transmitters from overwhelming
   the path of the packets between two IS-IS implementations.  This path
   typically includes a point-to-point link between two IS-IS neighbors
   which is usually over-sized compared to the capability of the IS-IS
   speakers, but potentially some internal elements inside each neighbor
   such as switching fabric, line card CPU, and forwarding plane buffers
   that may experience congestion.  These resources may be shared across
   multiple IS-IS adjacencies for the system and it is the
   responsibility of congestion control to ensure that these are shared
   reasonably.

   Reliability provides loss detection and recovery.  IS-IS already has
   mechanisms to ensure the reliable transmission of LSPs.  This is not
   changed by this document.

   The following two sections provide two Flow and/or Congestion control
   algorithms that may be implemented by taking advantage of the
   extensions defined in this document.  The signal that these IS-IS
   extensions defined in Section 4 and Section 5 provide are generic and
   are designed to support different sender-side algorithms.  A sender
   can unilaterally choose a different algorithm to use.

6.2.  Congestion and Flow Control algorithm

6.2.1.  Flow control

   A flow control mechanism creates a control loop between a single
   instance of a transmitter and a single receiver.  This section uses a
   mechanism similar to the TCP receive window to allow the receiver to
   govern the amount of data sent by the sender.  This receive window
   ('rwin') indicates an allowed number of LSPs that the sender may
   transmit before waiting for an acknowledgment.  The size of the
   receive window, in units of LSPs, is initialized with the value
   advertised by the receiver in the Receive Window sub-TLV.  If no

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   value is advertised, the transmitter should initialize rwin with its
   locally configured value for this neighbor.

   When the transmitter sends a set of LSPs to the receiver, it
   subtracts the number of LSPs sent from rwin.  If the transmitter
   receives a PSNP, then rwin is incremented for each acknowledged LSP.
   The transmitter must ensure that the value of rwin never goes
   negative.

   The RWIN value is of importance when the RTT is the limiting factor
   for the throughput.  In this case the optimal size is the desired LSP
   rate multiplied by the RTT.  The RTT being the addition of the link
   RTT plus the time taken by the receiver to acknowledge the first
   received LSP in its PSNP. 50 or 100 may be reasonable default
   numbers.  As an example, a RWIN of 100 requires a control plane input
   buffer of 150 kbytes per neighbor assuming an IS-IS MTU of 1500
   octets and limits the throughput to 10000 LSPs per second and per
   neighbor for a link RTT of 10 ms.  With the same RWIN, the throughput
   limitation is 2000 LSP per second when the RTT is 50ms.  That's the
   maximum throughput assuming no other limitations such as CPU
   limitations.

   Equally RTT is of importance for the performance.  That is why the
   performance improvements on the receiver specified in section
   Section 5 are important to achieve good throughput.  If the receiver
   does not support those performance improvements, in the worst case
   (small RWIN and high RTT) the throughput will be limited by the LSP
   Transmission Interval as defined in section Section 4.2.

6.2.1.1.  Operation on a point to point interface

   By sending the Receive Window sub-TLV, a node advertises to its
   neighbor its ability to receive that many un-acknowledged LSPs from
   the neighbor.  This is akin to a receive window or sliding window in
   flow control.  In some implementations, this value should reflect the
   IS-IS socket buffer size.  Special care must be taken to leave space
   for CSNPs and PSNPs and IIHs if they share the same input queue.  In
   this case, this document suggests advertising an LSP Receive Window
   corresponding to half the size of the IS-IS input queue.

   By advertising an LSP Transmission Interval sub-TLV, a node
   advertises its ability to receive LSPs separated by at least the
   advertised value, outside of LSP bursts.

   By advertising an LSP Burst Size sub-TLV, a node advertises its
   ability to receive that number of LSPs back-to-back.

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   The LSP transmitter MUST NOT exceed these parameters.  After having
   sent a full burst of LSPs, it MUST send the subsequent LSPs with a
   minimum of LSP Transmission Interval between LSP transmissions.  For
   CPU scheduling reasons, this rate MAY be averaged over a small
   period, e.g., 10-30ms.

   If either the LSP transmitter or receiver does not adhere to these
   parameters, for example because of transient conditions, this doesn't
   result in a fatal condition for IS-IS operation.  In the worst case,
   an LSP is lost at the receiver and this situation is already remedied
   by mechanisms in [ISO10589].  After a few seconds, neighbors will
   exchange PSNPs (for point-to-point interfaces) or CSNPs (for
   broadcast interfaces) and recover from the lost LSPs.  This worst
   case should be avoided as those additional seconds impact convergence
   time since the LSDB is not fully synchronized.  Hence it is better to
   err on the conservative side and to under-run the receiver rather
   than over-run it.

6.2.1.2.  Operation on a broadcast LAN interface

   Flow and congestion control on a LAN interface is out of scope for
   this document.

6.2.2.  Congestion Control

   Whereas flow control prevents the sender from overwhelming the
   receiver, congestion control prevents senders from overwhelming the
   network.  For an IS-IS adjacency, the network between two IS-IS
   neighbors is relatively limited in scope and includes a single link
   which is typically over-sized compared to the capability of the IS-IS
   speakers.  In situations where the probability of LSP drop is low,
   flow control Section 6.2.1 is expected to give good results, without
   the need to implement congestion control.  Otherwise, adding
   congestion control will help handling congestion of LSPs in the
   receiver.

   This section describes one sender-side congestion control algorithm
   largely inspired by the TCP congestion control algorithm [RFC5681].

   The proposed algorithm uses a variable congestion window 'cwin'.  It
   plays a role similar to the receive window described above.  The main
   difference is that cwin is adjusted dynamically according to various
   events described below.

6.2.2.1.  Core algorithm

   In its simplest form, the congestion control algorithm looks like the
   following:

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      +---------------+
      |               |
      |               v
      |   +----------------------+
      |   | Congestion avoidance |
      |   + ---------------------+
      |               |
      |               | Congestion signal
      ----------------+

                                  Figure 1

   The algorithm starts with cwin = cwin0 = LPP + 1.  In the congestion
   avoidance phase, cwin increases as LSPs are acked: for every acked
   LSP, cwin += 1 / cwin without exceeding RWIN.  When LSPs are
   exchanged, cwin LSPs will be acknowledged in 1 RTT, meaning cwin(t) =
   t/RTT + cwin0.  Since the RTT is low in many IS-IS deployments, the
   sending rate can reach fast rates in short periods of time.

   When updating cwin, it must not become higher than the number of LSPs
   waiting to be sent, otherwise the sending will not be paced by the
   receiving of acks.  Said differently, tx pressure is needed to
   maintain and increase cwin.

   When the congestion signal is triggered, cwin is set back to its
   initial value and the congestion avoidance phase starts again.

6.2.2.2.  Congestion signals

   The congestion signal can take various forms.  The more reactive the
   congestion signals, the fewer LSPs will be lost due to congestion.
   However, overly aggressive congestion signals will cause a sender to
   keep a very low sending rate even without actual congestion on the
   path.

   Two practical signals are given below.

   Delay: When receiving acknowledgements, a sender estimates the
   acknowledgement time of the receiver.  Based on this estimation, it
   can infer that a packet was lost, and infer congestion on the path.

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   There can be a timer per LSP, but this can become costly for
   implementations.  It is possible to use only a single timer t1 for
   all LSPs: during t1, sent LSPs are recorded in a list list_1.  Once
   the RTT is over, list_1 is kept and another list list_2 is used to
   store the next LSPs.  LSPs are removed from the lists when acked.  At
   the end of the second t1 period, every LSP in list_1 should have been
   acked, so list_1 is checked to be empty. list_1 can then be reused
   for the next RTT.

   There are multiple strategies to set the timeout value t1.  It should
   be based on measurements of the maximum acknowledgement time (MAT) of
   each PSNP.  The simplest one is to use three times the RTT.
   Alternatively an exponential moving average of the MATs, like
   [RFC6298].  A more elaborate one is to take a running maximum of the
   MATs over a period of a few seconds.  This value should include a
   margin of error to avoid false positives (e.g., estimated MAT measure
   variance) which would have a significant impact on performance.

   Loss: if the receiver has signaled the O-flag (Ordered
   acknowledgement) Section 4.4, a sender MAY record its sending order
   and check that acknowledgements arrive in the same order.  If not,
   some LSPs are missing and this MAY be used to trigger a congestion
   signal.

6.2.2.3.  Refinement

   With the algorithm presented above, if congestion is detected, cwin
   goes back to its initial value, and does not use the information
   gathered in previous congestion avoidance phases.

   It is possible to use a fast recovery phase once congestion is
   detected, to avoid going through this linear rate of growth from
   scratch.  When congestion is detected, a fast recovery threshold
   frthresh is set to frthresh = cwin / 2.  In this fast recovery phase,
   for every acked LSP, cwin += 1.  Once cwin reaches frthresh, the
   algorithm goes back to the congestion avoidance phase.

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      +---------------+
      |               |
      |               v
      |   +----------------------+
      |   | Congestion avoidance |
      |   + ---------------------+
      |               |
      |               | Congestion signal
      |               |
      |   +----------------------+
      |   |     Fast recovery    |
      |   +----------------------+
      |               |
      |               | frthresh reached
      ----------------+

                                  Figure 2

6.2.2.4.  Remarks

   This algorithm's performance is dependent on the LPP value.  Indeed,
   the smaller LPP is, the more information is available for the
   congestion control algorithm to perform well.  However, it also
   increases the resources spent on sending PSNPs, so a trade-off must
   be made.  This document recommends to use an LPP of 15 or less.  If a
   Receive Window is advertised, LPP SHOULD be lower and the best
   performance is achieved when LPP is an integer fraction of the
   Receive Window.

   Note that this congestion control algorithm benefits from the
   extensions proposed in this document.  The advertisement of a receive
   window from the receiver (Section 6.2.1) avoids the use of an
   arbitrary maximum value by the sender.  The faster acknowledgment of
   LSPs (Section 5.1) allows for a faster control loop and hence a
   faster increase of the congestion window in the absence of
   congestion.

6.2.3.  Pacing

   As discussed in [RFC9002], Section 7.7 a sender SHOULD pace sending
   of all in-flight LSPs based on input from the congestion controller.

   Sending multiple packets without any delay between them creates a
   packet burst that might cause short-term congestion and losses.
   Senders MUST either use pacing or limit such bursts.  Senders SHOULD
   limit bursts to LSP Burst Size.

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   Senders can implement pacing as they choose.  A perfectly paced
   sender spreads packets evenly over time.  For a window-based
   congestion controller, such as the one in this section, that rate can
   be computed by averaging the congestion window over the RTT.
   Expressed as an inter-packet interval in units of time:

   interval = (SRTT / cwin) / N

   SRTT is the smoothed round-trip time [RFC6298]

   Using a value for N that is small, but at least 1 (for example, 1.25)
   ensures that variations in RTT do not result in underutilization of
   the congestion window.

   Practical considerations, such as scheduling delays and computational
   efficiency, can cause a sender to deviate from this rate over time
   periods that are much shorter than an RTT.

   One possible implementation strategy for pacing uses a leaky bucket
   algorithm, where the capacity of the "bucket" is limited to the
   maximum burst size and the rate that the "bucket" fills is determined
   by the above function.

6.2.4.  Determining values to be advertised in the Flooding Parameters
        TLV

   The values that a receiver advertises do not need to be perfect.  If
   the values are too low then the transmitter will not use the full
   bandwidth or available CPU resources.  If the values are too high
   then the receiver may drop some LSPs during the first RTT and this
   loss will reduce the usable receive window and the protocol
   mechanisms will allow the adjacency to recover.  Flooding slower than
   both nodes can support will hurt performance, as will consistently
   overloading the receiver.

6.2.4.1.  Static values

   The values advertised need not be dynamic as feedback is provided by
   the acknowledgment of LSPs in SNP messages.  Acknowledgments provide
   a feedback loop on how fast the LSPs are processed by the receiver.
   They also signal that the LSPs can be removed from receive window,
   explicitly signaling to the sender that more LSPs may be sent.  By
   advertising relatively static parameters, we expect to produce
   overall flooding behavior similar to what might be achieved by
   manually configuring per-interface LSP rate-limiting on all
   interfaces in the network.  The advertised values could be based, for
   example, on offline tests of the overall LSP-processing speed for a
   particular set of hardware and the number of interfaces configured

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   for IS-IS.  With such a formula, the values advertised in the
   Flooding Parameters TLV would only change when additional IS-IS
   interfaces are configured.

   Static values are dependent on the CPU generation, class of router
   and network scaling, typically the number of adjacent neighbors.
   Examples at the time of publication are provided below.  LSP Burst
   Size could be in the range 5 to 20.  From a router perspective, this
   value typically depends on the queue(s) size(s) on the I/O path from
   the packet forwarding engine to the control plane which is very
   platform dependent.  It also depends upon how many IS-IS neighbors
   share this I/O path as typically all neighbors will send the same
   LSPs at the same time.  It may also depend on other incoming control
   plane traffic sharing that I/O path, how bursty they are, and how
   many incoming IS-IS packets are prioritized over other incoming
   control plane traffic.  As indicated in Section 3, the historical
   behavior from [ISO10589] allows a value of 10 hence 10 seems
   conservative.  From a network operation perspective, it would be
   beneficial for the burst size to be equal to or higher than the
   number of LSPs which may be originated by a single failure.  For a
   node failure, this is equal to the number of IS-IS neighbors of the
   failed node.  LSP Transmission Interval could be in the range of 1 ms
   to 33 ms.  As indicated in Section 3, the historical behavior from
   [ISO10589] is 33ms hence is conservative.  The LSP Transmission
   Interval is an advertisement of the receiver's sustainable LSP
   reception rate taking into account all aspects and in particular the
   control plane CPU and the I/O bandwidth.  It's expected to improve
   (hence decrease) as hardware and software naturally improve over
   time.  It should be chosen conservatively as this rate may be used by
   the sender in all conditions including the worst conditions.  It's
   also not a bottleneck as the flow control algorithm may use a higher
   rate in good conditions, in particular when the receiver acknowledges
   quickly and the receive window is large enough compared to the RTT.
   LPP could be in the range of 5 to 90 with a proposed 15.  A smaller
   value provides faster feedback at the cost of the small overhead of
   more PSNP messages.  PartialSNPInterval could be in the range 50ms to
   500ms with a proposed 200ms.  One may distinguish the value used
   locally from the value signaled to the sender.  The value used
   locally benefits from being small but is not expected to be the main
   parameter to improve performance.  It depends on how fast the IS-IS
   flooding process may be scheduled by the CPU.  It's safe as, even
   when the receiver CPU is busy, it will naturally delay its
   acknowledgments which provides a negative feedback loop.  The value
   advertised to the sender should be conservative (high enough) as this
   value could be used by the sender to send some LSPs rather than keep
   waiting for acknowledgments.  Receive Window in the range of 30 to
   200 with a proposed 60.  In general, the larger the better the
   performance on links with high RTT.  The higher the number and the

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   higher the number of IS-IS neighbors, the higher the use of control
   plane memory so it's mostly dependent on the amount of memory which
   may be dedicated to IS-IS flooding and the number of IS-IS neighbors.
   From a memory usage perspective, a priori, one could use the same
   value as the TCP receive window, but the value advertised should not
   be higher than the buffer of the "socket" used.

6.2.4.2.  Dynamic values

   The values may be updated dynamically, to reflect the relative change
   of load on the receiver, by improving the values when the receiver
   load is getting lower and degrading the values when the receiver load
   is getting higher.  For example, if LSPs are regularly dropped, or if
   the queue regularly comes close to being filled, then the values may
   be too high.  On the other hand, if the queue is barely used (by IS-
   IS), then the values may be too low.

   The values may also be absolute value reflecting relevant average
   hardware resources that are monitored, typically the amount of buffer
   space used by incoming LSPs.  In this case, care must be taken when
   choosing the parameters influencing the values in order to avoid
   undesirable or unstable feedback loops.  It would be undesirable to
   use a formula that depends, for example, on an active measurement of
   the instantaneous CPU load to modify the values advertised in the
   Flooding Parameters TLV.  This could introduce feedback into the IGP
   flooding process that could produce unexpected behavior.

6.2.5.  Operation considerations

   As discussed in Section 4.7, the solution is more effective on point-
   to-point adjacencies.  Hence a broadcast interface (e.g., Ethernet)
   only shared by two IS-IS neighbors should be configured as point-to-
   point in order to have more effective flooding.

6.3.  Transmitter Based Congestion Control Approach

   This section describes an approach to congestion control algorithm
   based on performance measured by the transmitter without dependance
   on signaling from the receiver.

6.3.1.  Router Architecture Discussion

   (The following description is an abstraction - implementation details
   vary.)

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   Existing router architectures may utilize multiple input queues.  On
   a given line card, IS-IS PDUs from multiple interfaces may be placed
   in a rate-limited input queue.  This queue may be dedicated to IS-IS
   PDUs or may be shared with other routing related packets.

   The input queue may then pass IS-IS PDUs to a "punt queue" which is
   used to pass PDUs from the data plane to the control plane.  The punt
   queue typically also has controls on its size and the rate at which
   packets will be punted.

   An input queue in the control plane may then be used to assemble PDUs
   from multiple linecards, separate the IS-IS PDUs from other types of
   packets, and place the IS-IS PDUs on an input queue dedicated to the
   IS-IS protocol.

   The IS-IS input queue then separates the IS-IS PDUs and directs them
   to an instance-specific processing queue.  The instance-specific
   processing queue may then further separate the IS-IS PDUs by type
   (IIHs, SNPs, and LSPs) so that separate processing threads with
   varying priorities may be employed to process the incoming PDUs.

   In such an architecture, it may be difficult for IS-IS in the control
   plane to determine what value should be advertised as a receive
   window.

   The following section describes an approach to congestion control
   based on performance measured by the transmitter without dependance
   on signaling from the receiver.

6.3.2.  Guidelines for transmitter side congestion controls

   The approach described in this section does not depend upon direct
   signaling from the receiver.  Instead it adapts the transmission rate
   based on measurement of the actual rate of acknowledgments received.

   Flow control is not used by this approach.  When congestion control
   is necessary, it can be implemented based on knowledge of the current
   flooding rate and the current acknowledgement rate.  The algorithm
   used is a local matter.  There is no requirement to standardize it
   but there are a number of aspects which serve as guidelines which can
   be described.  Algorithms based on this approach should follow the
   recommendations described below.

   A maximum LSP transmission rate (LSPTxMax) should be configurable.
   This represents the fastest LSP transmission rate which will be
   attempted.  This value should be applicable to all interfaces and
   should be consistent network wide.

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   When the current rate of LSP transmission (LSPTxRate) exceeds the
   capabilities of the receiver, the congestion control algorithm needs
   to quickly and aggressively reduce the LSPTxRate.  Slower
   responsiveness is likely to result in a larger number of
   retransmissions which can introduce much longer delays in
   convergence.

   Dynamic increase of the rate of LSP transmission (LSPTxRate) (i.e.,
   faster) should be done less aggressively and only be done when the
   neighbor has demonstrated its ability to sustain the current
   LSPTxRate.

   The congestion control algorithm should not assume the receive
   performance of a neighbor is static, i.e., it should handle transient
   conditions which result in a slower or faster receive rate on the
   part of a neighbor.

   The congestion control algorithm should consider the expected delay
   time in receiving an acknowledgment.  It therefore incorporates the
   neighbor partialSNPInterval (Section 4.5) to help determine whether
   acknowlegments are keeping pace with the rate of LSPs transmitted.
   In the absence of an advertisement of partialSNPInterval, a locally
   configured value can be used.

7.  IANA Considerations

7.1.  Flooding Parameters TLV

   IANA has made the following temporary allocation from the IS-IS TLV
   codepoint registry.  This document requests the allocation be made
   permanent.

        Type    Description                    IIH   LSP   SNP   Purge
        ----    ---------------------------    ---   ---   ---   ---
         21    Flooding Parameters TLV         y     n     y     n

                                  Figure 3

7.2.  Registry: IS-IS Sub-TLV for Flooding Parameters TLV

   This document creates the following sub-TLV Registry under the "IS-IS
   TLV Codepoints" grouping:

   Name: IS-IS Sub-TLVs for Flooding Parameters TLV.

   Registration Procedure(s): Expert Review

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   Expert(s): TBD

   Description: This registry defines sub-TLVs for the Flooding
   Parameters TLV(21).

   Reference: This document.

                   +=======+===========================+
                   |  Type | Description               |
                   +=======+===========================+
                   |   0   | Reserved                  |
                   +-------+---------------------------+
                   |   1   | LSP Burst Size            |
                   +-------+---------------------------+
                   |   2   | LSP Transmission Interval |
                   +-------+---------------------------+
                   |   3   | LSPs Per PSNP             |
                   +-------+---------------------------+
                   |   4   | Flags                     |
                   +-------+---------------------------+
                   |   5   | Partial SNP Interval      |
                   +-------+---------------------------+
                   |   6   | Receive Window            |
                   +-------+---------------------------+
                   | 7-255 | Unassigned                |
                   +-------+---------------------------+

                          Table 1: Initial Sub-TLV
                          allocations for Flooding
                               Parameters TLV

7.3.  Registry: IS-IS Bit Values for Flooding Parameters Flags Sub-TLV

   This document requests IANA to create a new registry, under the "IS-
   IS TLV Codepoints" grouping, for assigning Flag bits advertised in
   the Flags sub- TLV.

   Name: IS-IS Bit Values for Flooding Parameters Flags Sub-TLV.

   Registration Procedure: Expert Review

   Expert Review Expert(s): TBD

   Description: This registry defines bit values for the Flags sub-
   TLV(4) advertised in the Flooding Parameters TLV(21).

   Note: In order to minimize encoding space, a new allocation should
   pick the smallest available value.

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   Reference: This document.

               +=======+==================================+
               | Bit # | Description                      |
               +=======+==================================+
               |   0   | Ordered acknowledgement (O-flag) |
               +-------+----------------------------------+
               |  1-63 | Unassigned                       |
               +-------+----------------------------------+

                   Table 2: Initial bit allocations for
                              Flags Sub-TLV

8.  Security Considerations

   Security concerns for IS-IS are addressed in [ISO10589] , [RFC5304] ,
   and [RFC5310] .  These documents describe mechanisms that provide for
   the authentication and integrity of IS-IS PDUs, including SNPs and
   IIHs.  These authentication mechanisms are not altered by this
   document.

   With the cryptographic mechanisms described in [RFC5304] and
   [RFC5310] , an attacker wanting to advertise an incorrect Flooding
   Parameters TLV would have to first defeat these mechanisms.

   In the absence of cryptographic authentication, as IS-IS does not run
   over IP but directly over the link layer, it's considered difficult
   to inject false SNP/IIH without having access to the link layer.

   If a false SNP/IIH is sent with a Flooding Parameters TLV set to
   conservative values, the attacker can reduce the flooding speed
   between the two adjacent neighbors which can result in LSDB
   inconsistencies and transient forwarding loops.  However, it is not
   significantly different than filtering or altering LSPs which would
   also be possible with access to the link layer.  In addition, if the
   downstream flooding neighbor has multiple IGP neighbors, which is
   typically the case for reliability or topological reasons, it would
   receive LSPs at a regular speed from its other neighbors and hence
   would maintain LSDB consistency.

   If a false SNP/IIH is sent with a Flooding Parameters TLV set to
   aggressive values, the attacker can increase the flooding speed which
   can either overload a node or more likely generate loss of LSPs.
   However, it is not significantly different than sending many LSPs
   which would also be possible with access to the link layer, even with
   cryptographic authentication enabled.  In addition, IS-IS has
   procedures to detect the loss of LSPs and recover.

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   This TLV advertisement is not flooded across the network but only
   sent between adjacent IS-IS neighbors.  This would limit the
   consequences in case of forged messages, and also limits the
   dissemination of such information.

9.  Contributors

   The following people gave a substantial contribution to the content
   of this document and should be considered as coauthors:

   *  Jayesh J, Ciena, jayesh.ietf@gmail.com

   *  Chris Bowers, Juniper Networks, cbowers@juniper.net

   *  Peter Psenak, Cisco Systems, ppsenak@cisco.com

10.  Acknowledgments

   The authors would like to thank Henk Smit, Sarah Chen, Xuesong Geng,
   Pierre Francois, Hannes Gredler, Acee Lindem, Mirja Kuhlewind,
   Zaheduzzaman Sarker and John Scudder for their reviews, comments and
   suggestions.

   The authors would like to thank David Jacquet, Sarah Chen, and
   Qiangzhou Gao for the tests performed on commercial implementations
   and their identification of some limiting factors.

11.  References

11.1.  Normative References

   [ISO10589] ISO, "Intermediate system to Intermediate system intra-
              domain routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/
              IEC 10589:2002, Second Edition, November 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

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   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Chen, H., Jalil, L., and S. Dontula,
              "Dynamic Flooding on Dense Graphs", Work in Progress,
              Internet-Draft, draft-ietf-lsr-dynamic-flooding-18, 5
              April 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lsr-dynamic-flooding-18>.

   [RFC2973]  Balay, R., Katz, D., and J. Parker, "IS-IS Mesh Groups",
              RFC 2973, DOI 10.17487/RFC2973, October 2000,
              <https://www.rfc-editor.org/info/rfc2973>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/info/rfc9002>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

Appendix A.  Changes / Author Notes

   [RFC Editor: Please remove this section before publication]

   IND 00: Initial version.

   WG 00: No change.

   WG 01: IANA allocated code point.

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   WG 02: No change.

   WG 03:

   *  Pacing section added (taken from RFC 9002).

   *  Some text borrowed from RFC 9002 (QUIC Loss Detection and
      Congestion Control).

   *  Considerations on the special role of the DIS.

   *  Editorial changes.

   WG 04: Update IANA section as per IANA editor comments (2023-03-23).

   WG 06: AD review.

Authors' Addresses

   Bruno Decraene
   Orange
   Email: bruno.decraene@orange.com

   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA 95035
   United States of America
   Email: ginsberg@cisco.com

   Tony Li
   Juniper Networks, Inc.
   Email: tony.li@tony.li

   Guillaume Solignac
   Email: gsoligna@protonmail.com

   Marek Karasek
   Cisco Systems
   Pujmanove 1753/10a, Prague 4 - Nusle
   10 14000 Prague
   Czech Republic
   Email: mkarasek@cisco.com

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   Gunter Van de Velde
   Nokia
   Copernicuslaan 50
   2018 Antwerp
   Belgium
   Email: gunter.van_de_velde@nokia.com

   Tony Przygienda
   Juniper
   1137 Innovation Way
   Sunnyvale, Ca
   United States of America
   Email: prz@juniper.net

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