draft-ietf-isis-restart-00.txt   draft-ietf-isis-restart-01.txt 
Network Working Group M. Shand Network Working Group M. Shand
Internet Draft Cisco Systems Internet Draft Cisco Systems
Expiration Date: February 2002 Expiration Date: November 2002
Sept 2001 May 2002
Restart signaling for ISIS Restart signaling for ISIS
draft-ietf-isis-restart-00.txt draft-ietf-isis-restart-01.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Internet-Drafts are draft documents valid for a maximum of Drafts. Internet-Drafts are draft documents valid for a maximum of
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Adjacency re-acquisition is the first step in re-initialization. The Adjacency re-acquisition is the first step in re-initialization. The
restarting router explicitly notifies its neighbor that the restarting router explicitly notifies its neighbor that the
adjacency is being re-acquired, and hence that it should not re- adjacency is being re-acquired, and hence that it should not re-
initialize the adjacency. This is achieved by the inclusion of a new 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 "re-start" option (TLV) in the IIH PDU. The presence of this TLV
indicates that the sender supports the new restart capability and it indicates that the sender supports the new restart capability and it
carries flags that are used to convey information during a restart. carries flags that are used to convey information during a restart.
All IIHs transmitted by a router that supports this capability MUST All IIHs transmitted by a router that supports this capability MUST
include this TLV. include this TLV.
Type [TBD] Type 211
Length 3 Length 3
Value (3 octets) Value (3 octets)
Flags (1 octet) Flags (1 octet)
Bit 1 - Restart Request (RR) Bit 1 - Restart Request (RR)
Bit 2 - Restart Acknowledgment (RA) Bit 2 - Restart Acknowledgment (RA)
Bits 3-8 Reserved Bits 3-8 Reserved
Remaining Time (2 octets) Remaining Time (2 octets)
Remaining holding time (in seconds) Remaining holding time (in seconds)
(note: only required when RA bit is set) (note: only required when RA bit is set)
On receipt of an IIH with the "re-start" TLV having the RR bit set, On receipt of an IIH with the "re-start" TLV having the RR bit set,
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b) immediately (i.e. without waiting for any currently running timer 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 interval to expire, but with a small random delay of a few 10s of
milliseconds on LANs to avoid "storms"), transmit over the milliseconds on LANs to avoid "storms"), transmit over the
corresponding interface an IIH including the "re-start" TLV with corresponding interface an IIH including the "re-start" TLV with
the RR bit clear and the RA bit set, having updated the "Point-to- the RR bit clear and the RA bit set, having updated the "Point-to-
Point Adjacency State" option to reflect any new values received Point Adjacency State" option to reflect any new values received
from the re-starting router. (This allows the restarting router to from the re-starting router. (This allows the restarting router to
quickly acquire the correct information to place in its hellos.) quickly acquire the correct information to place in its hellos.)
The "Remaining Time" MUST be set to the current time (in seconds) The "Remaining Time" MUST be set to the current time (in seconds)
before the holding timer on this adjacency is due to expire, before the holding timer on this adjacency is due to expire. This
IIH SHOULD be transmitted before any LSPs or SNPs transmitted as a
result of the receipt of the original IIH.
c) if the corresponding interface is a Point-to-Point interface, or c) if the corresponding interface is a Point-to-Point interface, or
if the receiving router has the highest LnRouterPriority (with if the receiving router has the highest LnRouterPriority (with
highest source MAC address breaking ties) among those routers highest source MAC address breaking ties) among those routers
whose IIHs contain the restart TLV, excluding the transmitting whose IIHs contain the restart TLV, excluding the transmitting
router (note the actual DR is NOT changed by this process.), router (note the actual DR is NOT changed by this process.),
initiate the transmission over the corresponding interface of a initiate the transmission over the corresponding interface of a
complete set of CSNPs, and set SRMflags on the corresponding complete set of CSNPs, and set SRMflags on the corresponding
interface for all LSPs in the local LSP database. interface for all LSPs in the local LSP database.
Otherwise (i.e. if there was no adjacency to the system ID in Otherwise (i.e. if there was no adjacency in the "UP" state to the
question), process the IIH as normal by re-initializing the system ID in question), process the IIH as normal by re-initializing
adjacency, and setting the RA bit in the returned IIH. the adjacency, and setting the RA bit in the returned IIH.
A router that does not support the re-start capability will ignore A router that does not support the re-start capability will ignore
the "re-start" TLV and re-initialize the adjacency as normal, the "re-start" TLV and re-initialize the adjacency as normal,
returning an IIH without the "re-start" TLV. returning an IIH without the "re-start" TLV.
On starting, a router initializes the timer T3, starts timer T2 for On starting, a router initializes the timer T3, starts timer T2 for
each LSPDB and for each interface starts a timer T1 and transmits an each LSPDB and for each interface (and in the case of a LAN circuit,
IIH containing the "re-start" TLV with the RR bit set. for each level) 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 On a Point-to-Point circuit the "Point-to-Point Adjacency State"
Adjacency State" option with state "Down", and with empty SHOULD be set to "Init", because the receipt of the acknowledging
"Neighbor System ID" and "Neighbor Extended Local Circuit ID" IIH (with RA set) MUST cause the adjacency to enter "Up" state
options. The values of the "LocalCircuitID" and the "Extended immediately.
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 Transmission of "normal" IIHs is inhibited until the conditions
described below are met (in order to avoid causing an unnecessary described below are met (in order to avoid causing an unnecessary
adjacency re-initialization). On expiry of the timer T1, it is adjacency re-initialization). On expiry of the timer T1, it is
restarted and the IIH is re-transmitted as above. restarted and the IIH is re-transmitted as above.
On receipt of an IIH by the restarting router, a local adjacency is 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 established as usual, and if the IIH contains a "re-start" TLV with
the RA bit set, the receipt of the acknowledgement over that the RA bit set, the receipt of the acknowledgement over that
interface is noted (see section Error! Reference source not found.). interface is noted.
T3 is set to the minimum of its current value and the value of the T3 is set to the minimum of its current value and the value of the
"Remaining Time" field in the received IIH. "Remaining Time" field in the received IIH.
Receipt of an IIH not containing the "re-start" option is also Receipt of an IIH not containing the "re-start" option is also
treated as an acknowledgement, since it indicates that the neighbor treated as an acknowledgement, since it indicates that the neighbor
is not re-start capable. In this case the neighbor will have re- 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- initialized the adjacency as normal, which in the case of a Point-
to-Point link will guarantee that SRMflags have been set on its to-Point link will guarantee that SRMflags have been set on its
database, thus ensuring eventual LSPDB synchronization. In the case database, thus ensuring eventual LSPDB synchronization. In the case
of a LAN interface, the usual operation of the update process will of a LAN interface, the usual operation of the update process will
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match the values currently in use by the local system is ignored match the values currently in use by the local system is ignored
(since the IIH may have been transmitted before the neighbor had (since the IIH may have been transmitted before the neighbor had
received the new values from the re-starting router), but the received the new values from the re-starting router), but the
adjacency remains in the initializing state until the correct adjacency remains in the initializing state until the correct
information is received. information is received.
In the case of a LAN circuit the information in the Intermediate In the case of a LAN circuit the information in the Intermediate
Systems Neighbors option is recorded and used for the generation of Systems Neighbors option is recorded and used for the generation of
subsequent IIHs as normal. subsequent IIHs as normal.
When BOTH a complete set of CSNP(s) and an acknowledgement have been When BOTH a complete set of CSNP(s) (for each active level, in the
received over the interface, the timer T1 is cancelled. case of a pt-pt circuit) and an acknowledgement have been received
over the interface, the timer T1 is cancelled.
Once T3 has expired or been cancelled, subsequent IIHs are Once T3 has expired or been cancelled, subsequent IIHs are
transmitted according to the normal algorithms, but including the transmitted according to the normal algorithms, but including the
"re-start" TLV with both RR and RA clear. "re-start" TLV with both RR and RA clear.
If a LAN contains a mixture of systems, only some of which support If a LAN contains a mixture of systems, only some of which support
the new algorithm, database synchronization is still guaranteed, but the new algorithm, database synchronization is still guaranteed, but
the "old" systems will have re-initialized their adjacencies. the "old" systems will have re-initialized their adjacencies.
If an interface is active, but does not have any neighboring router If an interface is active, but does not have any neighboring router
reachable over that interface the timer T1 would never be cancelled, reachable over that interface the timer T1 would never be cancelled,
and according to clause 4.3.1.2 the SPF would never be run. and according to clause 4.3.1.2 the SPF would never be run.
Therefore timer T1 is cancelled after some pre-determined number of Therefore timer T1 is cancelled after some pre-determined number of
expirations. (By this time any existing adjacency on a remote system expirations (which MAY be 1). (By this time any existing adjacency
would probably have expired anyway.) on a remote system would probably have expired anyway.)
A router which supports re-start SHOULD ensure that the holding time A router which supports re-start SHOULD ensure that the holding time
of any IIHs it transmits is greater than the expected time to of any IIHs it transmits is greater than the expected time to
complete a re-start. complete a re-start. However, where this is impracticable or
undesirable a router MAY transmit one or more normal IIHs
(containing a restart option, but with RR and RA clear) after the
initial RR/RA exchange, but before synchronization has been
achieved, in order to extend the holding time of the neighbors
adjacencies, beyond that indicated in the remaining time field of
the neighbors IIH with the RA bit set.
4.2.1 Multiple levels
A router which is operating as both a level 1 and a level 2 router
on a particular interface MUST perform the above operations for each
level.
On a LAN interface, it MUST send and receive both Level 1 and
Level 2 IIHs and perform the CSNP synchronizations independently for
each level.
On a pt-pt interface, only as single IIH (indicating support for
both levels) is required, but it MUST perform the CSNP
synchronizations independently for each level.
4.3 Database synchronization 4.3 Database synchronization
When a router is started or re-started it can expect to receive a 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 (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 now guaranteed, since the "re-start" IIH with the RR bit set will be
retransmitted until the CSNP(s) are correctly received. retransmitted until the CSNP(s) are correctly received.
The CSNPs describe the set of LSPs that are currently held by each The CSNPs describe the set of LSPs that are currently held by each
neighbor. Synchronization will be complete when all these LSPs have neighbor. Synchronization will be complete when all these LSPs have
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In the case of a starting router, as soon as each adjacency is In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router's own zeroth established, and before any CSNP exchanges, the router's own zeroth
LSP is transmitted with the overload bit set. This prevents other LSP is transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will 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 occur if a previous copy of the routers LSP is still present in the
network) while any timer T2 is running. network) while any timer T2 is running.
When all the T2 timers have been cancelled, the own LSP(s) are When all the T2 timers have been cancelled, the own LSP(s) MAY be
regenerated with the overload bit clear (assuming the router isn't regenerated with the overload bit clear (assuming the router isn't
in fact overloaded), and flooded as normal. in fact overloaded, and there is no other reason, such as incomplete
BGP convergence, to keep the overload bit set), and flooded as
normal.
Other 'own' LSPs (including pseudonodes) are generated and flooded Other 'own' LSPs (including pseudonodes) are generated and flooded
as normal, irrespective of the timer T2. The SPF is also run as as normal, irrespective of the timer T2. The SPF is also run as
normal and the RIB and FIB updated as routes become available. normal and the RIB and FIB updated as routes become available.
4.3.1.2. Re-starting 4.3.1.2. Re-starting
In order to avoid causing unnecessary routing churn in other In order to avoid causing unnecessary routing churn in other
routers, it is highly desirable that the own LSPs generated by the routers, it is highly desirable that the own LSPs generated by the
restarting system are the same as those previously present in the restarting system are the same as those previously present in the
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pseudonode LSPs which it previously owned, but which are now no pseudonode LSPs which it previously owned, but which are now no
longer part of its set of pseudonode LSPs. longer part of its set of pseudonode LSPs.
If the timer T3 expires before all the T2 timers have expired, this If the timer T3 expires before all the T2 timers have expired, this
indicates that the synchronization process is taking longer than indicates that the synchronization process is taking longer than
minimum holding time of the neighbors. The router's own LSP(s) for minimum holding time of the neighbors. The router's own LSP(s) for
levels which have not yet completed their first SPF computation are levels which have not yet completed their first SPF computation are
then flooded with the overload bit set to indicate that the router's then flooded with the overload bit set to indicate that the router's
LSPDB is not yet synchronized (and other routers should therefore LSPDB is not yet synchronized (and other routers should therefore
not compute routes through this router). In order to prevent the not compute routes through this router). In order to prevent the
neighbor's adjacencies from expiring, IIHs are transmitted over all neighbor's adjacencies from expiring, IIHs with the normal interface
interfaces with neither RR nor RA set in the restart TLV. This will value for the holding time are transmitted over all interfaces with
cause the neighbors to refresh their adjacencies. The own LSP(s) neither RR nor RA set in the restart TLV. This will cause the
will continue to have the overload bit set until timer T2 has been neighbors to refresh their adjacencies. The own LSP(s) will continue
cancelled as in the case of starting for the first time described in to have the overload bit set until timer T2 has been cancelled as in
section 4.3.1.1 the case of starting for the first time described in section 4.3.1.1
5. Security Considerations 5. Security Considerations
This memo does not create any new security issues for the IS-IS This memo does not create any new security issues for the IS-IS
protocol. Security considerations for the base IS-IS protocol are protocol. Security considerations for the base IS-IS protocol are
covered in [2] and [3]. covered in [2] and [3].
6. References 6. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
skipping to change at page 9, line 27 skipping to change at page 9, line 47
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement 4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997 Levels", BCP 14, RFC 2119, March 1997
5 Katz, D., "Three-Way Handshake for IS-IS Point-to-Point 5 Katz, D., "Three-Way Handshake for IS-IS Point-to-Point
Adjacencies", draft-ietf-isis-3way-03.txt, July 2000 Adjacencies", draft-ietf-isis-3way-03.txt, July 2000
7. Acknowledgments 7. Acknowledgments
The author would like to acknowledge contributions made by Radia The author would like to acknowledge contributions made by Radia
Perlman, Mark Schaefer, Russ White and Rena Yang. Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, Russ White, and
Rena Yang.
8. Author's Addresses 8. Author's Address
Mike Shand Mike Shand
Cisco Systems Cisco Systems
4, The Square, 4, The Square,
Stockley Park, Stockley Park,
UXBRIDGE, UXBRIDGE,
Middlesex Middlesex
UB11 1BN, UK UB11 1BN, UK
Phone: +44 20 8756 8690 Phone: +44 208 824 8690
Email: mshand@cisco.com Email: mshand@cisco.com
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