Internet DRAFT - draft-decraeneginsberg-lsr-isis-fast-flooding
draft-decraeneginsberg-lsr-isis-fast-flooding
Network Working Group B. Decraene
Internet-Draft Orange
Intended status: Standards Track L. Ginsberg
Expires: 25 April 2022 Cisco Systems
T. Li
Arista Networks
G. Solignac
M. Karasek
Cisco Systems
C. Bowers
Juniper Networks, Inc.
G. Van de Velde
Nokia
P. Psenak
Cisco Systems
T. Przygienda
Juniper
22 October 2021
IS-IS Fast Flooding
draft-decraeneginsberg-lsr-isis-fast-flooding-00
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 25 April 2022.
Copyright Notice
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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 Window sub-TLV . . . . . . . . . . . . . . . . 6
4.2. LSP Transmission Interval sub-TLV . . . . . . . . . . . . 6
4.3. LSPs Per PSNP sub-TLV . . . . . . . . . . . . . . . . . . 6
4.4. Flags sub-TLV . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Partial SNP Interval sub-TLV . . . . . . . . . . . . . . 7
4.6. Operation on a LAN interface . . . . . . . . . . . . . . 7
5. Performance improvement on the receiver . . . . . . . . . . . 8
5.1. Rate of LSP Acknowledgments . . . . . . . . . . . . . . . 8
5.2. Packet Prioritization on Receive . . . . . . . . . . . . 9
6. Congestion and Flow Control . . . . . . . . . . . . . . . . . 10
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Congestion and Flow Control algorithm: Example 1 . . . . 10
6.3. Congestion Control algorithm: Example 2 . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Changes / Author Notes . . . . . . . . . . . . . . . 22
Appendix B. Issues for Further Discussion . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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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 both CPU speeds
and interface speeds were much slower and the scale of an area was
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 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 minimumLSPTransmission-Interval.
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 an order of magnitude similar to 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 a series of rapid 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 of use to the peer in support of faster
flooding.
Type: TBD1
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 Window sub-TLV
The LSP Burst Window sub-TLV advertises the maximum number of LSPs
that the node can receive with no separation interval between LSPs.
Type: 1
Length: 4 octets
Value: number of LSPs that can be sent 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
received on this interface, after the maximum number of un-
acknowledged LSPs has been sent.
Type: 2
Length: 4 octets
Value: minimum interval, in micro-seconds, between two consecutive
LSPs sent after the burst window has been used
The LSP Transmission Interval is an advertisement of the receiver's
steady-state LSP reception rate.
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 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
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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 is set, the LSP 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 trigger faster retransmission of 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 unacknowldeged 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. 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, copies of which
will be received by all transmitters. The flooding parameters sent
by the LSP receiver MUST be understood as instructions from the
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receiver to each 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 a 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 carry 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 Window values, it should use
the smallest value.
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, PSNP PDUs 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 in a single PSNP.
Faster LSP flooding benefits from a faster feedback loop. This
requires a reduction in the delay in sending PSNPs.
The receiver SHOULD reduce its partialSNPInterval. 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 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 LPP SHOULD also be less than or
equal to 90 as this is the maximum number of LSPs that can be
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acknowledged in a PSNP at common MTU sizes, hence waiting longer
would not reduce the number of PSNPs sent but would delay the
acknowledgements. Based on experimental evidence, 15 unacknowledged
LSPs is a good value assuming that the LSP Burst Window is at least
30 and reasonably fast CPUs for both the transmitter and receiver.
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 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 have yet to be transmitted. It may also make it more likely
that a receiver becomes overwhelmed by LSP transmissions.
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It is therefore RECOMMENDED that implementations prioritize the
receipt of Hellos and then SNPs over LSPs. Implementations MAY also
prioritize IS-IS packets over other less critical protocols.
6. Congestion and Flow Control
6.1. Overview
Ensuring the goodput between two entities is a layer 4 responsibility
as per the OSI model and a typical example is the TCP protocol
defined in RFC 793 [RFC0793] and relies on the flow control,
congestion control, and reliability mechanisms of the protocol.
Flow control creates a control loop between a transmiter and a
receiver so that the transmitter does not overwhelm the receiver.
TCP provides a mean for the receiver to govern the amount of data
sent by the sender through the use of a sliding window.
Congestion control creates multiple interacting control loops between
multiple transmitters and multiple receivers to prevent the
transmitters from overwhelming the overall network. For an IS-IS
adjacency, the network between two IS-IS neighbors is relatively
limited in scope and consist of a link that is typically over-sized
compared to the capability of the IS-IS speakers, but may also
includes components inside both routers such as a 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 provides examples of Flow and/or
Congestion control algorithms as examples that may be implemented by
taking advantage of the extensions defined in this document. They
are non-normative. An implementation may implement any congestion
control algorithm.
6.2. Congestion and Flow Control algorithm: Example 1
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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 example 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 LSP Burst Window sub-TLV. If no
value is advertised, the transmitter should initialize rwin with its
own local value.
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.
6.2.1.1. Operation on a point to point interface
By sending the LSP Burst Window sub-TLV, a node advertises to its
neighbor its ability to receive that many un-acknowledged LSPs from
the neighbor, with no separation interval. 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 CSNP and PSNP (SNP) PDUs and IIHs if
they share the same input queue. In this case, this document
suggests advertising an LSP Burst 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.
The LSP transmitter MUST NOT exceed these parameters. After having
sent a full burst of un-acknowledged LSPs, it MUST send the following
LSPs with an LSP Transmission Interval between LSP arrivals. For CPU
scheduling reasons, this rate may be averaged over a small period
e.g. 10 to 30ms.
If either the LSP transmitter or receiver does not adhere to these
parameters, for example because of transient conditions, this causes
no fatal condition to the operation of IS-IS. 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
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case should be avoided as those additional seconds impact convergence
time as 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
In order for the LSP Burst Window to be a useful parameter, an LSP
transmitter needs to be able to keep track of the number of un-
acknowledged LSPs it has sent to a given LSP receiver. On a LAN
there is no explicit acknowledgment of the receipt of LSPs between a
given LSP transmitter and a given LSP receiver. However, an LSP
transmitter on a LAN can infer whether any LSP receiver on the LAN
has requested retransmission of LSPs from the DIS by monitoring PSNPs
generated on the LAN. If no PSNPs have been generated on the LAN for
a suitable period of time, then an LSP transmitter can safely set the
number of un-acknowledged LSPs to zero. Since this suitable period
of time is much higher than the fast acknowledgment of LSPs defined
in Section 5.1, the sustainable transmission rate of LSPs will be
much slower on a LAN interface than on a point to point interface.
The LSP Burst Window is still very useful for the first burst of LSPs
sent, especially in the case of a single node failure that requires
the flooding of a relatively small number of LSPs.
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 congestion control algorithm largely
inspired by the TCP congestion control algorithm RFC 5681 [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 dynamically changed 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 := LPP + 1. In the congestion
avoidance phase, cwin increases as LSPs are acked: for every acked
LSP, cwin += 1 / cwin. Thus, the sending rate roughly increases
linearly with the RTT. 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 less LSPs will be lost due to congestion.
However, congestion signals too aggressive will cause a sender to
keep a very low sending rate even without actual congestion on the
path.
Two practical signals are given hereafter.
Timers: 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
every 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 measures of the maximum acknowledgement time (MAT) of
each PSNPs. The simplest one is to use a exponential moving average
of the MATs, like RFC 6298 [RFC6298]. A more elaborate one is to
take a running maximum of the MATs over a period of time 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.
Reordering: a sender can record its sending order and check that
acknowledgements arrive on the same order than LSPs. This makes an
additional assumption and should ideally be backed up by a
confirmation by the receiver that this assumption stands. The O flag
defined in Section 4.4 serves this purpose.
6.2.2.3. Refinement 1
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. Refinement 2
The rates of increase were inspired from TCP RFC 5681 [RFC5681], but
it is possible that a different rate of increase for cwin in the
congestion avoidance phase actually yields better results due to the
low RTT values in most IS-IS deployments.
6.2.2.5. 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 tradeoff must be
made. This document recommends to use an LPP of 15 or less. If an
LSP Burst Window is advertised, LPP SHOULD be lower and the best
performance is achieved when LPP is an integer fraction of the LSP
Burst 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.
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6.2.3. 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 several
orders of magnitude slower than both nodes can achieve 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 may be based, for
example, on an 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 of 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 values may be too low.
The values may also be absolute value reflecting relevant average
hardware resources that are been 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 instable 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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6.2.4. Operation considerations
As discussed in Section 4.6, the solution is more effective on point
to point adjacencies. Hence a broadcast interface (e.g. Ethernet)
only shared by two IS-IS neighbhors should be configured as point to
point in order to have a more effective flooding.
6.3. Congestion Control algorithm: Example 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-ISs PDU from other types of
packets, and place the IS-IS PDUs in 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 specififc
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 Flow Control
The congestion control algorithm described in this section does not
depend upon direct signaling from the receiver. Instead it adapts
the tranmsmission rate based on measurement of the actual rate of
acknowledgments received.
When flow control is necessary, it can be implemented in a
straightforward manner 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 target 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 flow control algorithm needs to
aggressively reduce the LSPTxRate within a few seconds. Slower
responsiveness is likely to result in a large number of
retransmissions which can introduce much larger delays in
convergence.
NOTE: Even with modest increases in flooding speed (for example, a
target LSPTxMax of 300 LSPs/second (10 times the typical rate
supported today)), a topology change triggering 2100 new LSPs would
only take 7 seconds to complete.
Dynamic adjustment of the rate of LSP transmission (LSPTxRate)
upwards (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 flow control algorithm MUST NOT assume the receive capabilities
of a neighbor are static, i.e., it MUST handle transient conditions
which result in a slower or faster receive rate on the part of a
neighbor.
The flow control algorithm needs to 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.
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7. IANA Considerations
IANA is requested to allocate one TLV from the IS-IS TLV codepoint
registry.
Type Description IIH LSP SNP Purge
---- --------------------------- --- --- --- ---
TBD1 Flooding Parameters TLV y n y n
Figure 3
This document creates the following sub-TLV Registry:
Name: Sub-TLVs for TLV TBD1 (Flooding Parameters TLV).
Registration Procedure(s): Expert Review
Expert(s): TBD
Reference: TBD
+=======+===========================+
| Type | Description |
+=======+===========================+
| 0 | Reserved |
+-------+---------------------------+
| 1 | LSP Burst Window |
+-------+---------------------------+
| 2 | LSP Transmission Interval |
+-------+---------------------------+
| 3 | LSPs Per PSNP |
+-------+---------------------------+
| 4 | Flags |
+-------+---------------------------+
| 5 | Partial SNP Interval |
+-------+---------------------------+
| 6-255 | Unassigned |
+-------+---------------------------+
Table 1: Initial allocations
This document also requests IANA to create a new registry for
assigning Flag bits advertised in the Flags sub-TLV.
Name: Flooding Parameters Flags Bits.
Registration Procedure:
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Expert Review Expert(s): TBD
+--------+------------------------+
| Bit # | Description |
+--------|------------------------+
| 0 | O Flag |
+--------|------------------------+
8. Security Considerations
Security concerns for IS-IS are addressed in [ISO10589] , [RFC5304] ,
and [RFC5310] . These documents describe mechanisms that provide 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/IHH 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.
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.
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9. Contributors
The following people gave a substantial contribution to the content
of this document and should be considered as coauthors:
Acee Lindem, Cisco Systems, acee@cisco.com
Jayesh J, Juniper Networks, jayeshj@juniper.net
10. Acknowledgments
The authors would like to thank Henk Smit, Sarah Chen, Xuesong Geng,
Pierre Francois and Hannes Gredler 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] International Organization for Standardization,
"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>.
[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>.
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[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., Ginsberg, L., Chen, H., Przygienda,
T., Cooper, D., Jalil, L., Dontula, S., and G. S. Mishra,
"Dynamic Flooding on Dense Graphs", Work in Progress,
Internet-Draft, draft-ietf-lsr-dynamic-flooding-09, 9 June
2021, <https://www.ietf.org/archive/id/draft-ietf-lsr-
dynamic-flooding-09.txt>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
Appendix A. Changes / Author Notes
[RFC Editor: Please remove this section before publication]
00: Initial version.
Appendix B. Issues for Further Discussion
[RFC Editor: Please remove this section before publication]
This section captures issues which the authors either have not yet
had time to address or on which the authors have not yet reached
consensus. Future revisions of this document may include new/altered
text relevant to these issues.
There are no open issues at this time.
Authors' Addresses
Bruno Decraene
Orange
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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
Arista Networks
5453 Great America Parkway
Santa Clara, California 95054
United States of America
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
Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
United States of America
Email: cbowers@juniper.net
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Gunter Van de Velde
Nokia
Copernicuslaan 50
2018 Antwerp
Belgium
Email: gunter.van_de_velde@nokia.com
Peter Psenak
Cisco Systems
Apollo Business Center Mlynske nivy 43
821 09 Bratislava
Slovakia
Email: ppsenak@cisco.com
Tony Przygienda
Juniper
1137 Innovation Way
Sunnyvale, Ca
United States of America
Email: prz@juniper.net
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