Network Working Group M. Shand
Internet Draft Cisco Systems
Expiration Date: August 2001
February 2001
Restart signaling for ISIS
draft-shand-isis-restart-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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1. Abstract
The IS-IS routing protocol (RFC 1142 [2], ISO/IEC 10589 [3]) is a
link state intra-domain routing protocol. Normally, when an IS-IS
router is re-started, the neighboring routers detect the restart
event and cycle their adjacencies with the restarting router through
the down state. This is necessary in order to invoke the protocol
mechanisms to ensure correct re-synchronization of the LSP database.
However, the cycling of the adjacency state causes the neighbors to
regenerate their LSPs describing the adjacency concerned. This in
turn causes temporary disruption of routes passing through the
restarting router.
In certain scenarios such temporary disruption of the routes is
highly undesirable.
This draft describes a mechanism for a restarting router to signal
that it is restarting to its neighbors, and allow them to re-
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establish their adjacencies without cycling through the down state,
while still correctly initiating database synchronization.
When such a router is restarted, it is highly desirable that it does
not re-compute its own routes until it has achieved database
synchronization with its neighbors. Re-computing its routes before
synchronization is achieved will result in its own routes being
temporarily incorrect.
This draft additionally describes a mechanism for a restarting
router to determine when it has achieved synchronization with its
neighbors.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [4].
3. Overview
There are two related problems with the existing specification of
IS-IS with regard to re-synchronization of LSP databases when a
router is re-started.
Firstly, when a routing process restarts, and an adjacency to a
neighboring router is re-initialized the neighboring routing process
does three things
1. It re-initializes the adjacency and causes its own LSP(s) to be
regenerated, thus triggering SPF runs throughout the domain.
2. It sets SRMflags on its own LSP database on the adjacency
concerned.
3. In the case of a Point-to-Point link it transmits a (set of)
CSNP(s) over the adjacency.
In the case of a restarting router process, the first of these is
highly undesirable, but the second is essential in order to ensure
re-synchronization of the LSP database.
Secondly, whether or not the router is being re-started, it is
desirable to be able to determine when the LSP databases of the
neighboring routers have been synchronized (so that the overload bit
can be cleared in the router's own LSP, for example). This document
describes modifications to achieve this.
It is assumed that the three-way handshake [5] is being used on
Point-to-Point circuits.
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4. Approach
4.1 Adjacency re-acquisition
Adjacency re-acquisition is the first step in re-initialization. The
restarting router explicitly notifies its neighbor that the
adjacency is being re-acquired, and hence that it should not re-
initialize the adjacency. This is achieved by the inclusion of a new
"re-start" option (TLV) in the IIH PDU. The presence of this TLV
indicates that the sender supports the new restart capability and it
carries flags that are used to convey information during a restart.
All IIHs transmitted by a router that supports this capability MUST
include this TLV.
Type [TBD]
Length 1
Value (1 octet)
Bit 1 - Restart Request (RR)
Bit 2 - Restart Acknowledgment (RA)
Bits 3-8 - Reserved
On receipt of an IIH with the "re-start" TLV having the RR bit set,
if there exists on this interface an adjacency in state "Up" with
the same System ID, and in the case of a LAN circuit, with the same
source LAN address, then, irrespective of the other contents of the
"Intermediate System Neighbors" option (LAN circuits), or the
"Point-to-Point Adjacency State" option (Point-to-Point circuits):-
a) Refresh the timer on the adjacency and leave the adjacency in
state "Up",
b) immediately (i.e. without waiting for any currently running timer
interval to expire, but with a small random delay of a few 10s of
milliseconds on LANs to avoid "storms"), transmit over the
corresponding interface an IIH including the "re-start" TLV with
the RR bit clear and the RA bit set, having updated the "Point-to-
Point Adjacency State" option to reflect any new values received
from the re-starting router. (This allows the restarting router to
quickly acquire the correct information to place in its hellos.),
c) if the corresponding interface is a Point-to-Point interface, or
i) if the receiving router is the LAN level n designated router
(where n is the level of the IIH), or
ii) if the transmitting router is currently (according to the
receiving router) the LAN level n designated router (where n is
the level of the IIH) and the receiving router would be elected
the LAN level n designated router if the transmitting router
were ignored (note the actual DR is NOT changed by this
process),
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initiate the transmission over the corresponding interface of a
complete set of CSNPs, and set SRMflags on the corresponding
interface for all LSPs in the local LSP database.
Otherwise (i.e. if there was no adjacency to the system ID in
question), process the IIH as normal by re-initializing the
adjacency, and setting the RA bit in the returned IIH.
A router that does not support the re-start capability will ignore
the "re-start" TLV and re-initialize the adjacency as normal,
returning an IIH without the "re-start" TLV.
On starting, a router starts a timer T1 and transmits an IIH
containing the "re-start" TLV with the RR bit set.
1. On a LAN circuit the IIH contains an empty "Intermediate
Systems Neighbors" TLV.
2. On a Point-to-Point circuit the IIH contains a "Point-to-Point
Adjacency State" option with state "Initializing", and with
empty "Neighbor System ID" and "Neighbor Extended Local
Circuit ID" options. The values of the "LocalCircuitID" and
the "Extended Local CircuitID" may, but need not be, the same
as those used previously for this circuit.
Transmission of "normal" IIHs is inhibited until the conditions
described below are met (in order to avoid causing an unnecessary
adjacency re-initialization). On expiry of the timer T1, it is
restarted and the IIH is retransmitted as above.
On receipt of an IIH by the restarting router, a local adjacency is
established as usual, and if the IIH contains a "re-start" TLV with
the RA bit set, the receipt of the acknowledgement over that
interface is noted.
Receipt of an IIH not containing the "re-start" option is also
treated as an acknowledgement, since it indicates that the neighbor
is not re-start capable. In this case the neighbor will have re-
initialized the adjacency as normal, which in the case of a Point-
to-Point link will guarantee that SRMflags have been set on its
database. In the case of LAN interface, the usual operation of the
update process will ensure that synchronization is eventually
achieved.
In the case of a Point-to-Point circuit, the "LocalCircuitID" and
"Extended Local Circuit ID" information contained in the IIH can be
used immediately to generate an IIH containing the correct 3-way
handshake information. The presence of "Neighbor System ID" or
"Neighbor Extended Local Circuit ID" information which does not
match the values currently in use by the local system is ignored
(since the IIH may have been transmitted before the neighbor had
received the new values from the re-starting router), but the
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adjacency remains in the initializing state until the correct
information is received.
In the case of a LAN circuit the information in the Intermediate
Systems Neighbors option is recorded and used for the generation of
subsequent IIHs as normal.
When BOTH a complete set of CSNP(s) and an acknowledgement have been
received over the interface, the timer T1 is cancelled.
Once T1 has been cancelled, subsequent IIHs are transmitted
according to the normal algorithms, but including the "re-start" TLV
with both RR and RA clear.
If a LAN contains a mixture of systems, only some of which support
the new algorithm, database synchronization is still guaranteed, but
the "old" systems will have re-initialized their adjacencies.
If an interface is active, but does not have any neighboring router
reachable over that interface the timer T1 would never be cancelled,
and according to clause 4.2.1.2 the SPF would never be run.
Therefore timer T1 is cancelled after some pre-determined
expirations. (By this time any existing adjacency on a remote system
would probably have expired anyway.)
4.1.1 State Table
The above operations can be summarized by the following state table.
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Event | Running | Restarting | Seen RA | Seen CSNP
==================================================================
RX RR | Set SRM | Set SRM | Set SRM | Set SRM
| Send RA | Send RA | Send RA | Send RA
| Send CSNP | Send CSNP | Send CSNP | Send CSNP
-------------+------------+------------+------------+-------------
RX RA | | Goto Seen | | Cancel T1
| | RA | | Goto Running
-------------+------------+------------+------------+-------------
RX CSNP | | Goto Seen | Cancel T1 |
| | CSNP | Goto |
| | | Running |
-------------+------------+------------+------------+-------------
RX IIH | | Cancel T1 | Cancel T1 | Cancel T1
with no | | Goto | Goto | Goto
Reset TLV | | Running | Running | Running
-------------+------------+------------+------------+-------------
T1 | | Send RR | Send RR | Send RR
Expires | | Send CSNP | Send CSNP | Send CSNP
| | Start T1 | Start T1 | Start T1
-------------+------------+------------+------------+-------------
T1 | | Cancel T1 | Cancel T1 | Cancel T1
Expires | | Goto | Goto | Goto
n times | | Running | Running | Running
-------------+------------+------------+------------+-------------
Router | Set SRM | Set SRM | Set SRM | Set SRM
Restarted | Send RR | Send RR | Send RR | Send RR
| Send CSNP | Send CSNP | Send CSNP | Send CSNP
| Start T1 | Start T1 | Start T1 | Start T1
| Goto | Goto | Goto | Goto
| Restarting | Restarting | Restarting | Restarting
==================================================================
4.2 Database synchronization
When a router is started or re-started it can expect to receive a
(set of) CSNP(s) over each interface. The arrival of the CSNP(s) is
now guaranteed, since the "re-start" IIH with the RR bit set will be
retransmitted until the CSNP(s) are correctly received.
The CSNPs describe the set of LSPs that are currently held by each
neighbor. Synchronization will be complete when all these LSPs have
been received.
On starting, a router starts a timer T2 for each active level. In
addition to normal processing of the CSNPs, the set of LSPIDs
contained in the first complete set of CSNP(s) received over each
interface is recorded. If there are multiple interfaces on the
restarting router, the recorded set of LSPIDs is the union of those
received over each interface. LSPs with a remaining lifetime of zero
are NOT so recorded.
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As LSPs are received (by the normal operation of the update process)
over any interface, the corresponding LSPID entry is removed (it is
also removed if the LSP had arrived before the CSNP containing the
reference). When the list of LSPIDs becomes empty, the timer T2 is
cancelled.
At this point the local database is guaranteed to contain all the
LSP(s) (either the same sequence number, or a more recent sequence
number) which were present in the neighbors' databases at the time
of re-starting. LSPs that arrived in a neighbor's database after the
time of re-starting may, or may not, be present, but the normal
operation of the update process will guarantee that they will
eventually be received. At this point the local database is deemed
to be "synchronized".
Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime
are not recorded, it is unlikely that cancellation of the timer T2
will be prevented by waiting for an LSP which will never arrive.
However it is possible that an LSP with a small remaining lifetime
was placed in the list. If the re-synchronization process takes
longer than 'ZeroAgeLifetime' (default 60 seconds), the
corresponding LSP will have been removed from the neighbors' LSP
databases and will never arrive. Under these circumstances, the
timer T2 may expire, and the databases are deemed to be
synchronized.
4.2.1 LSP generation and flooding and SPF computation
The operation of a router starting, as opposed to re-starting is
somewhat different. These two cases are dealt with separately below.
4.2.1.1. Starting for the first time
In the case of a starting router, the router's own zeroth LSP is
first transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will
occur if a previous copy of the routers LSP is still present in the
network) while the timer T2 is running.
When the timer T2 expires, or is cancelled, the own LSP is
regenerated with the overload bit clear (assuming the router isn't
in fact overloaded), and flooded as normal.
Other 'own' LSPs (including pseudonodes) are generated and flooded
as normal, irrespective of the timer T2. The SPF is also run as
normal and the RIB and FIB updated as routes become available.
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4.2.1.2. Re-starting
In order to avoid causing unnecessary routing churn in other
routers, it is highly desirable that the own LSPs generated by the
restarting system are the same as those previously present in the
network (assuming no other changes have taken place). It is
important therefore not to regenerate and flood the LSPs until all
the adjacencies have been re-established and any information
required for propagation into the local LSPs is fully available.
Ideally, the information should be loaded into the LSPs in a
deterministic way, such that the same information occurs in the same
place in the same LSP (and hence the LSPs are identical to their
previous versions). If this can be achieved, the new versions will
not even cause SPF to be run in other systems. However, provided the
same information is included in the set of LSPs (albeit in a
different order, and possibly different LSPs), the result of running
the SPF will be the same and will not cause churn to the forwarding
tables.
In the case of a re-starting router, none of the router's own non-
pseudonode LSPs are transmitted, nor is the SPF run to update the
forwarding tables while the timer T2 is running, or while the timer
T1 is still running on any of the interfaces.
Redistribution of inter-level information must be regenerated before
this router's LSP is flooded to other nodes. Therefore the level-n
non-pseudonode LSP(s) should not be flooded until the other level's
T2 timer has expired and its SPF has been run. This ensures that any
inter-level information that should be propagated can be included in
the level-n LSP(s).
During this period, if one of the router's own (including
pseudonodes) LSPs is received, which the local router does not
currently have in its own database, it is NOT purged. Under normal
operation, such an LSP would be purged, since the LSP clearly should
not be present in the global LSP database. However, in the present
circumstances, this would be highly undesirable, because it could
cause premature removal of an own LSP -- and hence churn in remote
routers. Even if the local system has one or more own LSPs (which is
has generated, but not yet transmitted) it is still not valid to
compare the received LSP against this set, since it may be that as a
result of propagation between level 1 and level 2 (or vice versa) a
further own LSP will need to be generated when the LSP databases
have synchronized.
When the timer T2 expires, or is cancelled, the SPF is run to update
the RIB and FIB.
Once the other level's SPF has run and any inter-level propagation
has been resolved, the 'own' LSPs can be generated and flooded. Any
'own' LSPs which were previously ignored, but which are not part of
the current set of 'own' LSPs (including pseudonodes) should then be
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purged. Note that it is possible that a Designated Router change may
have taken place, and consequently the router should purge those
pseudonode LSPs which it previously owned, but which are now no
longer part of its set of pseudonode LSPs.
5. Security Considerations
This memo does not create any new security issues for the IS-IS
protocol. Security considerations for the base IS-IS protocol are
covered in [2] and [3].
6. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
December 1990.
3 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 10589:1992.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
5 Katz, D., "Three-Way Handshake for IS-IS Point-to-Point
Adjacencies", draft-ietf-isis-3way-03.txt, July 2000
7. Acknowledgments
The author would like to acknowledge contributions made by Radia
Perlman, Mark Schaefer, Russ White and Rena Yang.
8. Author's Addresses
Mike Shand
Cisco Systems
4, The Square,
Stockley Park,
UXBRIDGE,
Middlesex
UB11 1BN, UK
Phone: +44 20 8756 8690
Email: mshand@cisco.com
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