In theory, there is no difference between theory and practice.
But in practice, there is.
Jan L.A. van de Snepscheut
Interior Gateway Protocols such as IS-IS are designed to provide
timely information about the best routes in a routing domain. The
original design of IS-IS, as described in ISO 10589 [1] has proved to
be quite durable. However, a number of original design choices have
been modified. This document addresses differences between the
protocol described in ISO 10589 and the protocol that can be observed
on the wire today. A companion document discusses differences
between the protocol described in RFC 1195 [2] for routing IP traffic
and current practice.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are to be interpreted as described in RFC 2119 [3].
Some parameters that were defined as constant in ISO 10589 are
modified in practice. These include the following
(1) MaxAge - the lifetime of a Link State PDU (LSP)
(2) ISISHoldingMultiplier - a parameter used to describe the
generation of hello packets
(3) ReceiveLSPBufferSize - discussed in a later section
Each LSP contains a RemainingLifetime field which is initially set to
the MaxAge value on the generating IS. The value stored in this
field is decremented to mark the passage of time and the number of
times it has been forwarded. When the value of a foreign LSP becomes
0, an IS initiates a purging process which will flush the LSP from
the network. This ensures that corrupted or otherwise invalid LSPs
do not remain in the network indefinitely. The rate at which LSPs
are regenerated by the originating IS is determined by the value of
maximumLSPGenerationInterval.
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MaxAge is defined in ISO 10589 as an Architectural constant of 20
minutes, and it is recommended that maximumLSPGenerationInterval be
set to 15 minutes. These times have proven to be too short in some
networks, as they result in a steady flow of LSP updates even when
nothing is changing. To reduce the rate of generation, some
implementations allow these times to be set by the network operator.
The relation between MaxAge and maximumLSPGenerationInterval is
discussed in section 7.3.21 of ISO 10589. If MaxAge is smaller than
maximumLSPGenerationInterval, then an LSP will expire before it is
replaced. Further, as RemainingLifetime is decremented each time it
is forwarded, an LSP far from its origin appears older and is removed
sooner. To make sure that an LSP survives long enough to be
replaced, MaxAge should exceed maximumLSPGenerationInterval by at
least ZeroAgeLifetime + minimumLSPTransmissionInterval. The first
term, ZeroAgeLifetime, is an estimate of how long it takes to flood
an LSP through the network. The second term,
minimumLSPTransmissionInterval, takes into account how long a router
might delay before sending an LSP. The original recommendation was
that MaxAge be at least 5 minutes larger than
maximumLSPGenerationInterval, and that recommendation is still valid
today.
An implementation MAY use a value of MaxAge that is greater than 1200
seconds. MaxAge SHOULD exceed maximumLSPGenerationInterval by at
least 300 seconds. An implementation SHOULD NOT use its value of
MaxAge to discard LSPs from peers, as discussed below.
An implementation is not required to coordinate the RemainingLifetime
it assigns to LSPs to the RemainingLifetime values it accepts, and
MUST ignore the following sentence from section 7.3.16.3. of ISO
10589.
"If the value of Remaining Lifetime [of the received LSP] is
greater than MaxAge, the LSP shall be processed as if there
were a checksum error."
An IS sends IS to IS Hello Protocol Data Units (IIHs) on a periodic
basis over active circuits, allowing other attached routers to
monitor their aliveness. The IIH includes a two byte field called
the Holding Time which defines the time to live of an adjacency. If
an IS does not receive a hello from an adjacent IS within this
holding time, the adjacent IS is assumed to be no longer operational,
and the adjacency is removed.
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ISO 10589 defines ISISHoldingMultiplier to be 10, and states that the
value of Holding Time should be ISISHoldingMultiplier multiplied by
iSISHelloTimer for ordinary systems, and dRISISHelloTimer for a DIS.
This implies that the neighbor must lose 10 IIHs before an adjacency
times out.
In practice, a value of 10 for the ISISHoldingMultiplier has proven
to be too large. DECnet PhaseV defined two related values. The
variable holdingMultiplier, with a default value of 3, was used for
point-to-point IIHs, while the variable ISISHoldingMultiplier, with a
default value of 10, was used for LAN IIHs. Most implementations
today set the default ISISHoldingMultiplier to 3 for both circuit
types.
Note that adjacent systems may use different values for Holding Time
and will form an adjacency with non-symmetric hold times.
An implementation MAY allow ISISHoldingMultiplier to be configurable.
Values lower than 3 are unstable, and may cause adjacencies to flap.
Some values that were defined as variables in ISO 10589 do not vary
in practice. These include
(1) ID Length - the length of the SystemID
(2) maximumAreaAddresses
(3) Protocol Version
The ID Length is a field carried in all PDUs. The ID Length defines
the length of the System ID, and is allowed to take values from 0 to
8. A value of 0 is interpreted to define a length of 6 bytes. As
suggested in B.1.1.3 of [1], it is easy to use an Ethernet MAC
address to generate a unique 6 byte System ID. Since the SystemID
only has significance within the IGP Domain, 6 bytes has proved to be
easy to use and ample in practice. There are also new IS-IS Traffic
Engineering TLVs which assume a 6 byte System ID. Choices for the ID
length other than 6 are difficult to support today. Implementations
may interoperate without being able to deal with System IDs of any
length other than 6.
An implementation MUST use an ID Length of 6, and MUST check the ID
Length defined in the IS-IS PDUs it receives. If a router encounters
a PDU with an ID Length different from 0 or 6, section 7.3.15.a.2
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dictates that it MUST discard the PDU, and SHOULD generate an
appropriate notification. ISO 10589 defines the notification
iDFieldLengthMismatch, while the IS-IS MIB [7] defines the
notification isisIDLenMismatch.
The value of maximumAreaAddresses is defined to be an integer between
1 and 254, and defines the number of synonymous Area Addresses that
can be in use in an L1 area. This value is advertised in the header
of each IS-IS PDU.
Most deployed networks use one Area Address for an L1 area. When
merging or splitting areas, a second address is required for seamless
transition. The third area address was originally required to
support DECnet PhaseIV addresses as well as OSI addresses during a
transition.
ISO 10589 requires that all Intermediate Systems in an area or domain
use a consistent value for maximumAreaAddresses. Common practice is
for an implementation to use the value 3. Therefore an
implementation that only supports 3 can expect to interoperate
successfully with other conformant systems.
ISO 10589 specifies that an advertised value of 0 is treated as
equivalent to 3, and that checking the value for consistency may be
omitted if an implementation only supports the value 3.
An implementation SHOULD use the value 3, and it SHOULD check the
value advertised in IS-IS PDUs it receives. If a router receives a
PDU with maximumAreaAddresses that is not 0 or 3, it MUST discard the
PDU, as described in section 7.3.15.a.3, and it SHOULD generate an
appropriate notification. ISO 10589 defines the notification
maximumAreaAddressMismatch, while the IS-IS MIB [7] defines the
notification isisMaxAreaAddressesMismatch.
IS-IS PDUs include two one-byte fields in the headers:
"Version/Protocol ID Extension" and "Version".
An implementation SHOULD set both fields to 1, and it SHOULD check
the values of these fields in IS-IS PDUs it receives. If a router
receives a PDU with a value other than 1 for either field, it MUST
drop the packet, and SHOULD generate the isisVersionSkew
notification.
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Section 7.2.2, ISO 10589 describes four metrics: Default Metric,
Delay Metric, Expense Metric, and Error Metric. None but the Default
Metric are used in deployed networks, and most implementations only
consider the Default Metric. In ISO 10589, the most significant bit
of the 8 bit metrics was the field S (Supported), used to define if
the metric was meaningful.
If this IS does not support this metric it shall set bit S to 1
to indicate that the metric is unsupported.
The Supported bit was always 0 for the Default Metric, which must
always be supported. However, RFC 2966 [5] uses this bit in the
Default Metric to mark L1 routes that have been leaked from L1 to L2
and back down into L1 again.
Implementations MUST generate the Default Metric when using narrow
metrics, and SHOULD ignore the other three metrics when using narrow
metrics. Implementations MUST assume that the Default Metric is
supported, even if the S bit is set. RFC 2966 describes restrictions
on leaking such routes learned from L1 into L2.
Since IS-IS does not allow segmentation of protocol PDUs, Link State
PDUs (LSPs) must be propagated without modification on all IS-IS
enabled links throughout the area/domain. Thus it is essential to
configure a maximum size that all routers can forward, receive, and
store.
This affects three aspects, which we discuss in turn:
(1) The largest LSP we can receive (ReceiveLSPBufferSize)
(2) The size of the largest LSP we can generate
(originatingL1LSPBufferSize and
originatingL2LSPBufferSize)
(3) Available Link MTU for supported Circuits (MTU). Note
this often differs from the MTU available to IP clients.
ISO 10589 defines the architectural constant ReceiveLSPBufferSize
with value 1492 bytes, and two private management parameters,
originatingL1LSPBufferSize for level 1 PDUs and
originatingL2LSPBufferSize for level 2 PDUs. The originating buffer
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size parameters define the maximum size of an LSP that a router can
generate. ISO 10589 directs the implementor to treat a PDU larger
than ReceiveLSPBufferSize as an error.
It is crucial that
originatingL1LSPBufferSize <= ReceiveLSPBufferSize
originatingL2LSPBufferSize <= ReceiveLSPBufferSize
and that for all L1 links in the area
originatingL1LSPBufferSize <= MTU
and for all L2 links in the domain
originatingL2LSPBufferSize <= MTU
The original thought was that operators could decrease the
originating Buffer size when dealing with smaller MTUs, but would not
need to increase ReceiveLSPBufferSize beyond 1492.
With the definition of new information to be advertised in LSPs, such
as the Traffic Engineering TLVs, the limited space of the LSP
database which may be generated by each router (256 * 1492 bytes at
each level) has become an issue. Given that modern networks with
MTUs larger than 1492 on all links are not uncommon, one method which
can be used to expand the LSP database size is to allow values of
ReceiveLSPBufferSize greater than 1492.
Allowing ReceiveLSPBUfferSize to become a configurable parameter
rather than an architectural constant must be done with care: if any
system in the network does not support values larger than 1492 or one
or more link MTUs used by IS-IS anywhere in the area/domain is
smaller than the largest LSP which may be generated by any router,
then full propagation of all LSPs may not be possible, resulting in
routing loops and black holes.
The steps below are recommended when changing ReceiveLSPBufferSize.
(1) Set the ReceiveLSPBufferSize to a consistent value throughout
the network.
(2) The implementation MUST not enable IS-IS on circuits which do
not support an MTU at least as large as the originating
BufferSize at the appropriate level.
(3) Include an originatingLSPBufferSize TLV when generating LSPs,
introduced in section 9.8 of ISO 10589:2002 [1].
(4) When receiving LSPs, check for an originatingLSPBufferSize
TLV, and report the receipt of values larger than the local
value of ReceiveLSPBufferSize through the defined
Notifications and Alarms.
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(5) Report the receipt of a PDU larger than the local
ReceiveLSPBufferSize through the defined Notifications and
Alarms.
(6) Do not discard large PDUs by default. Storing and processing
them as normal PDUs may help maintain coherence in a
misconfigured network.
Steps 1 and 2 are enough by themselves, but the consequences of
mismatch are serious enough and difficult enough to detect, that
steps 3-6 are recommended to help track down and correct problems.
To prevent the establishment of adjacencies between systems which may
not be able to successfully receive and propagate IS-IS PDUs due to
inconsistent settings for originatingLSPBufferSize and
ReceiveLSPBufferSize, section 8.2.3 of [1] requires padding on
point-to-point links.
On point-to-point links, the initial IIH is to be padded to the
maximum of
(1) Link MTU
(2) originatingL1LSPBufferSize if the link is to be used for L1
traffic
(3) originatingL2LSPBufferSize if the link is to be used for L2
traffic
In section 6.7.2 e) ISO 10589 assumes
Provision that failure to deliver a specific subnetwork SDU
will result in the timely disconnection of the subnetwork
connection in both directions and that this failure will be
reported to both systems
With this service provided by the link layer, the requirement that
only the initial IIH be padded was sufficient to check the
consistency of the MTU on the two sides. If the PDU was too big to
be received, the link would be reset. However, link layer protocols
in use on point-to-point circuits today often lack this service, and
the initial padded PDU might be silently dropped without resetting
the circuit. Therefore, the requirement that only the initial IIH be
padded does not provide the guarantees anticipated in ISO 10589.
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If an implementation is using padding to detect problems, point-to-
point IIH PDUs SHOULD be padded until the sender declares an
adjacency on the link to be in state Up. If the implementation
implements RFC 3373 [4], "Three-Way Handshake for IS-IS Point-to-
Point Adjacencies" then this is when the three-way state is Up: if
the implementation use the "classic" algorithm described in ISO
10589, this is when adjacencyState is Up. Transmission of padded IIH
PDUs SHOULD be resumed whenever the adjacency is torn down, and
SHOULD continue until the sender declares the adjacency to be in
state Up again.
If an implementation is using padding, and originatingL1LSPBUfferSize
or originatingL2LSPBUfferSize is modified, adjacencies SHOULD be
brought down and reestablished so the protection provided by padding
IIH PDUs is performed consistent with the modified values.
Some implementations choose not to pad. Padding does not solve all
problems of misconfigured systems. In particular, it does not
provide a transitive relation. Assume that A, B, and C all pad IIH
PDUs, that A and B can establish an adjacency, and that B and C can
establish an adjacency. We still cannot conclude that A and C could
establish an adjacency, if they were neighbors.
The presence or absence of padding TLVs MUST NOT be one of the
acceptance tests applied to a received IIH regardless of the state of
the adjacency.
A checksum of 0 is impossible if the checksum is computed according
to the rules of ISO 8473 [8].
ISO 10589, section 7.3.14.2(i), states:
A Link State PDU received with a zero checksum shall be treated
as if the Remaining Lifetime were zero. The age, if not zero,
shall be overwritten with zero.
That is, ISO 10589 directs the receiver to purge the LSP. This has
proved to be disruptive in practice. An implementation SHOULD treat
all LSPs with a zero checksum and a non-zero remaining lifetime as if
they had as checksum error. Such packets SHOULD be discarded.
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While ISO 10589 requires in section 7.3.14.2 e) that any LSP received
with an invalid PDU checksum should be purged, this has been found to
be disruptive. Most implementations today follow the revised
specification, and simply drop the LSP.
In ISO 10589:2002 [1], Section 7.3.14.2, it states:
(e) An Intermediate system receiving a Link State PDU with an
incorrect LSP Checksum or with an invalid PDU syntax SHOULD
1) generate a corruptedLSPReceived circuit event,
2) discard the PDU.
In section 8.2.4.2, ISO 10589 does not explicitly require comparison
of the source ID of a received IIH with the neighbourSystemID
associated with an existing adjacency on a point-to-point link.
To address this omission, implementations receiving an IIH PDU on a
point to point circuit with an established adjacency SHOULD check the
Source ID field and compare that with the neighbourSystemID of the
adjacency. If these differ, an implementation SHOULD delete the
adjacency.
Given that IIH PDUs as specified in ISO 10589 do not include a
check-sum, it is possible that a corrupted IIH may falsely indicate a
change in the neighbor's System ID. The required subnetwork
guarantees for point-to-point links, as described in 6.7.2 g) 1)
assume that undetected corrupted PDUs are very rare (one event per
four years). A link with frequent errors that produce corrupted data
could lead to flapping an adjacency. Inclusion of an optional
checksum TLV as specified in "Optional Checksums in IS-IS" [6], may
be used to detect such corruption. Hello packets carrying this TLV
that are corrupted PDUs SHOULD be silently dropped, rather than
dropping the adjacency.
Some implementations have chosen to discard received IIHs where the
source ID differs from the neighbourSystemID. This may prevent
needless flapping caused by undetected PDU corruption. If an actual
administrative change to the neighbor's system ID has occurred, using
this strategy may require the existing adjacency to timeout before an
adjacency with the new neighbor can be established. This is
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expedited if the neighbor resets the circuit as anticipated in 10589
after a System ID change, or resets the 3-way adjacency state, as
anticipated in RFC 3373.
When an Intermediate System shuts down, it may leave old LSPs in the
network. In the normal course of events, a rebooting system flushes
out these old LSPs by reissuing those fragments with a higher
sequence number, or by purging fragments that it is not currently
generating.
In the case where a received LSP or SNP entry and an LSP in the local
database have the same LSP ID, same sequence number, non-zero
remaining lifetimes, but different non-zero checksums, the rules
defined in [1] cannot determine which of the two is "newer". In this
case, an implementation may opt to perform an additional test as a
tie breaker by comparing the checksums. Implementations that elect
to use this method MUST consider the LSP/SNP entry with the higher
checksum as newer. When comparing the checksums the checksum field
is treated as a 16 bit unsigned integer in network byte order (i.e.,
most significant byte first).
The choice of higher checksum, rather than lower, while arbitrary,
aligns with existing implementations and ensures compatibility.
Note that a purged LSP (i.e., an LSP with remaining lifetime set to
0) is always considered newer than a non-purged copy of the same LSP.
There are a number of cases in which a complete set of CSNPs must be
generated. The DIS on a LAN, two IS's peering over a P2P link, and
an IS helping another IS perform graceful restart must generate a
complete set of CSNPs to assure consistent LSP Databases throughout.
Section 7.3.15.3 of [1] defines a complete set of CSNPs to be:
"A complete set of CSNPs is a set whose Start LSPID and End
LSPID ranges cover the complete possible range of LSPIDs.
(i.e., there is no possible LSPID value which does not appear
within the range of one of the CSNPs in the set). "
Strict adherence to this definition is required to ensure the
reliability of the update process. Deviation can lead to subtle and
hard to detect defects. It is not sufficient to send a set of CSNPs
which merely cover the range of LSPIDs which are in the local
database. The set of CSNPs must cover the complete possible range of
LSPIDs.
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Consider the following example:
If the current Level 1 LSP database on a router consists of the
following non pseudo-node LSPs:
From system 1111.1111.1111 LSPs numbered 0-89(59H)
From system 2222.2222.2222 LSPs numbered 0-89(59H)
If the maximum size of a CSNP is 1492 bytes, then 90 CSNP entries can
fit into a single CSNP PDU. The following set of CSNP start/end
LSPIDs constitute a correctly formatted complete set:
Start LSPID End LSPID
0000.0000.0000.00-00 1111.1111.1111.00-59
1111.1111.1111.00-5A FFFF.FFFF.FFFF.FF-FF
The following are examples of incomplete sets of CSNPS:
Start LSPID End LSPID
0000.0000.0000.00-00 1111.1111.1111.00-59
1111.1111.1111.00-5A 2222.2222.2222.00-59
The sequence above has a gap after the second entry.
Start LSPID End LSPID
0000.0000.0000.00-00 1111.1111.1111.00-59
2222.2222.2222.00-00 FFFF.FFFF.FFFF.FF-FF
The sequence above has a gap between the first and second entry.
Although it is legal to send a CSNP which contains no actual LSP
entry TLVs, it should never be necessary to do so in order to conform
to the specification.
To deal with transient problems that prevent an IS from storing all
the LSPs it receives, ISO 10589 defines an LSP Database Overload
condition in section 7.3.19. When an IS is in Database Overload
condition, it sets a flag called the Overload Bit in the non-
pseudonode LSP number Zero that it generates. Section 7.2.8.1 of ISO
10589 instructs other systems not to use the overloaded IS as a
transit router. Since the overloaded IS does not have complete
information, it may not be able to compute the right routes, and
routing loops could develop.
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An overloaded router might become the DIS. An implementation SHOULD
not set the Overload bit in PseudoNode LSPs that it generates, and
Overload bits seen in PseudoNode LSPs SHOULD be ignored.
The clarifications in this document do not raise any new security
concerns, as there is no change in the underlying protocol described
in ISO 10589 [1].
[1] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002.
[2] Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
December 1990.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Katz, D. and Saluja, R., " Three-Way Handshake for Intermediate
System to Intermediate System (IS-IS) Point-to-Point
Adjacencies", RFC 3373, September 2002.
[5] Li, T., Przygienda, T. and H. Smit, "Domain-wide Prefix
Distribution with Two-Level IS-IS", RFC 2966, October 2000.
[6] Koodli, R. and R. Ravikanth, "Optional Checksums in Intermediate
System to Intermediate System (ISIS)", RFC 3358, August 2002.
[7] Parker, J., "Management Information Base for IS-IS", Work in
Progress, January 2004.
[8] ITU, "Information technology - Protocol for providing the
connectionless-mode network service", ISO/IEC 8473-1, 1998.
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This document is the work of many people, and is the distillation of
over a thousand mail messages. Thanks to Vishwas Manral, who pushed
to create such a document. Thanks to Danny McPherson, the original
editor, for kicking things off. Thanks to Mike Shand, for his work
in creating the protocol, and his uncanny ability to remember what
everything is for. Thanks to Micah Bartell and Philip Christian, who
showed us how to document difference without displaying discord.
Thanks to Les Ginsberg, Neal Castagnoli, Jeff Learman, and Dave Katz,
who spent many hours educating the editor. Thanks to Radia Perlman,
who is always ready to explain anything. Thanks to Satish Dattatri,
who was tenacious in seeing things written up correctly. Thanks to
Russ White, whose writing improved the treatment of every topic he
touched. Thanks to Shankar Vemulapalli, who read several drafts with
close attention. Thanks to Don Goodspeed, for his close reading of
the text. Thanks to Aravind Ravikumar, who pointed out that we
should check Source ID on point-to-point IIH packets. Thanks to
Michael Coyle for identifying the quotation from Jan L.A. van de
Snepscheut. Thanks for Alex Zinin's ministrations behind the scenes.
Thanks to Tony Li and Tony Przygienda, who kept us on track as the
discussions veered into the weeds. And thanks to all those who have
contributed, but whose names I have carelessly left from this list.
Jeff Parker
Axiowave Networks
200 Nickerson Road
Marlborough, Mass 01752
USA
EMail: jparker@axiowave.com
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