Internet DRAFT - draft-ietf-lsr-isis-fast-flooding
draft-ietf-lsr-isis-fast-flooding
Network Working Group B. Decraene
Internet-Draft Orange
Intended status: Experimental L. Ginsberg
Expires: 18 August 2024 Cisco Systems
T. Li
Juniper Networks, Inc.
G. Solignac
M. Karasek
Cisco Systems
G. Van de Velde
Nokia
T. Przygienda
Juniper
15 February 2024
IS-IS Fast Flooding
draft-ietf-lsr-isis-fast-flooding-07
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 18 August 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . 7
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 1 . . . . . . . . . 11
6.3. Congestion Control algorithm 2 . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7.1. Flooding Parameters TLV . . . . . . . . . . . . . . . . . 20
7.2. Registry: IS-IS Sub-TLV for Flooding Parameters TLV . . . 20
7.3. Registry: IS-IS Bit Values for Flooding Parameters Flags
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 23
11.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Changes / Author Notes . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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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.
Historically, flooding rates have been conservative - on the order of
10s of LSPs/second. This is the result of guidance in the base
specification [ISO10589] 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.
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.
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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.
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:
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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 similar as the 33 ms
value.
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
change in value since the last transmission. For a parameter which
has never been advertised, an IS SHOULD use its local default value.
That value SHOULD be configurable on a per-node basis and MAY be
configurable on a per-interface basis.
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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
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.
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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| ...
+-+-+-+-+-+-+-+-+...
When the O-flag (Ordered acknowledgement) is set, the LSPs will be
acknowledged in the order they are 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.
4.6. Receive Window sub-TLV
The Receive Window (RWIN) sub-TLV advertises the maximum number of
unacknowledged LSPs that the node can receive.
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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.
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.
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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
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.
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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 1
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 ko 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.
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.
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.
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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.
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:
+---------------+
| |
| v
| +----------------------+
| | Congestion avoidance |
| + ---------------------+
| |
| | Congestion signal
----------------+
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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.
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.
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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.
+---------------+
| |
| 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.
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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.
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.
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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.
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
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.
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.
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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. Congestion Control algorithm 2
This section describes a 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.)
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 accurately track the state of the various input queues and
determine what value should be advertised as a current receive
window.
The following section describes a congestion control algorithm based
on performance measured by the transmitter without dependance on
signaling from the receiver.
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6.3.2. Transmitter Based Congestion Control
The congestion control algorithm 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.
When congestion control is necessary, it can be implemented based on
knowledge of the current flooding rate and the current
acknowledgement rate. Such an algorithm is a local matter and there
is no requirement or intent to standardize an algorithm. There are a
number of aspects which serve as guidelines which can be described.
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.
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 MUST NOT assume the receive
performance of a neighbor is static, i.e., it MUST 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
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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
Expert(s): TBD
Description: This registry defines sub-TLVs for the Flooding
Parameters TLV(21).
Reference: This document.
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+=======+===========================+
| 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.
Reference: This document.
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+=======+==================================+
| 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 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-16, 14
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lsr-dynamic-flooding-16>.
[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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