it is restarted and the IIH is retransmitted with both RR and SA bits
set (only the RR bit has changed state from earlier IIHs).
Upon receipt of an IIH with the RR bit set (regardless of whether the
SA is set or not), the behavior described in 3.2.1 is followed.
When an IIH is received by the starting router and the IIH contains a
restart TLV with the RA bit set (and on LAN circuits with a Restart
Neighbor System ID which matches that of the local system), the
receipt of the acknowledgement over that interface is noted.
On a Point-to-Point link, receipt of an IIH not containing the
restart TLV is also treated as an acknowledgement, since it indicates
that the neighbor is not restart capable. Since the neighbor will
have reinitialized the adjacency, this guarantees that SRMflags have
been set on its database, thus ensuring eventual LSPDB
synchronization. However, since no CSNP is guaranteed to be received
over this interface, the timer T1 is cancelled immediately without
waiting for a complete set of CSNP(s). Synchronization may therefore
be deemed complete even though there are some LSPs which are held
(only) by this neighbor (see section 3.4).
In the case of a LAN interface, receipt of an IIH not containing the
restart TLV is unremarkable since synchronization can still occur so
long as at least one of the non-restarting neighboring routers on the
LAN supports restart. Therefore T1 continues to run in this case.
If none of the neighbors on the LAN are restart capable, T1 will
eventually expire after the locally defined number of retries. The
usual operation of the update process will ensure that
synchronization is eventually achieved.
When BOTH a complete set of CSNP(s) (for each active level, in the
case of a point-to-point circuit) and an acknowledgement have been
received over the interface, the timer T1 is cancelled. Subsequent
IIHs sent by the starting router have the RR and RA bits clear and
the SA bit set in the restart TLV.
Timer T1 is cancelled after some pre-determined number of expirations
(which MAY be 1).
When the T2 timer(s) are cancelled or expire, transmission of
"normal" IIHs (with RR, RA, and SA bits clear) will begin.
3.3.3. 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 point-to-point interface, only a single IIH (indicating support
for both levels) is required, but it MUST perform the CSNP
synchronizations independently for each level.
3.4. Database Synchronization
When a router is started or restarted it can expect to receive a (set
of) CSNP(s) over each interface. The arrival of the CSNP(s) is now
guaranteed, since an 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.
When (re)starting, a router starts an instance of timer T2 for each
LSPDB as described in 3.3.1 or 3.3.2. 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,
together with their remaining lifetime. In the case of a LAN
interface, a complete set of CSNPs MUST consist of CSNPs received
from neighbor(s) which are not restarting. If there are multiple
interfaces on the (re)starting 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.
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 an LSP arrives before the CSNP containing the
reference). When an LSPID has been held in the list for its
indicated remaining lifetime, it is removed from the list. When the
list of LSPIDs is empty and the timer T1 has been cancelled for all
the interfaces that have an adjacency at this level, 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) that 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, and those with a short remaining lifetime are
deleted from the list when the lifetime expires, cancellation of the
timer T2 will not be prevented by waiting for an LSP that will never
arrive.
3.4.1. LSP Generation and Flooding and SPF Computation
The operation of a router starting, as opposed to restarting, is
somewhat different. These two cases are dealt with separately below.
3.4.1.1. Restarting
In order to avoid causing unnecessary routing churn in other routers,
it is highly desirable that the router’s 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 is 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 may 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 restarting router, none of the router’s own LSPs are
transmitted, nor are the router’s own forwarding tables updated while
the timer T3 is running.
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) MUST 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 which is to 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 a router’s own LSP - and hence churn in
remote routers. Even if the local system has one or more of the
router’s own LSPs (which it 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 router’s own LSP will need to be
generated when the LSP databases have synchronized.
During this period a restarting router SHOULD send CSNPs as it
normally would. Information about the router’s own LSPs MAY be
included, but if it is included it MUST be based on LSPs which have
been received, not on versions which have been generated (but not yet
transmitted). This restriction is necessary to prevent premature
removal of an LSP from the global LSP database.
When the timer T2 expires or is cancelled indicating that
synchronization for that level is complete, the SPF for that level is
run in order to derive any information which is required to be
propagated to another level, but the forwarding tables are not yet
updated.
Once the other level’s SPF has run and any inter-level propagation
has been resolved, the router’s 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) MUST
then be 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.
When all the T2 timers have expired or been cancelled, the timer T3
is cancelled and the local forwarding tables are updated.
If the timer T3 expires before all the T2 timers have expired or been
cancelled, this indicates that the synchronization process is taking
longer than the minimum holding time of the neighbors. The router’s
own LSP(s) for levels which have not yet completed their first SPF
computation are then flooded with the overload bit set to indicate
that the router’s LSPDB is not yet synchronized (and therefore other
routers MUST NOT compute routes through this router). Normal
operation of the update process resumes and the local forwarding
tables are updated. In order to prevent the neighbor’s adjacencies
from expiring, IIHs with the normal interface value for the holding
time are transmitted over all interfaces with neither RR nor RA set
in the restart TLV. This will cause the neighbors to refresh their
adjacencies. The router’s own LSP(s) will continue to have the
overload bit set until timer T2 has expired or been cancelled.
3.4.1.2. Starting
In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router’s own zeroth
LSP is 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 router’s own zeroth LSP is still present in
the network) while any timer T2 is running.
When all the T2 timers have been cancelled, the router’s own LSP(s)
MAY be regenerated with the overload bit clear (assuming the router
is not 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 LSPs owned by this router (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.
To avoid the possible formation of temporary blackholes, the starting
router sets the SA bit in the restart TLV (as described in 3.3.2) in
all IIHs that it sends.
When all T2 timers have been cancelled, the starting router MUST
transmit IIHs with the SA bit clear.
4. State Tables
This section presents state tables which summarize the behaviors
described in this document. Other behaviors, in particular adjacency
state transitions and LSP database update operation, are NOT included
in the state tables except where this document modifies the behaviors
described in [2] and [4].
The states named in the columns of the tables below are a mixture of
states that are specific to a single adjacency (ADJ suppressed, ADJ
Seen RA, ADJ Seen CSNP) and states which are indicative of the state
of the protocol instance (Running, Restarting, Starting, SPF Wait).
Three state tables are presented from the point of view of a running
router, a restarting router, and a starting router.
4.1. Running Router
Event | Running | ADJ suppressed
==============================================================
RX RR | Maintain ADJ State |
| Send RA |
| Set SRM,send CSNP |
| (Note 1) |
| Update Hold Time, |
| set Restart Mode |
| (Note 2) |
-------------+----------------------+-------------------------
RX RR clr | Clr Restart mode |
-------------+----------------------+-------------------------
RX SA | Suppress IS neighbor |
| TLV in LSP(s) |
| Goto ADJ Suppressed |
-------------+----------------------+-------------------------
RX SA clr | |Unsuppress IS neighbor
| | TLV in LSP(s)
| |Goto Running
==============================================================
Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c
Note 2: If Restart Mode clear
4.2. Restarting Router
Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait
| | RA | CSNP |
===================================================================
Router | Send IIH/RR | | |
restarts | ADJ Init | | |
| Start T1,T2,T3 | | |
------------+--------------------+-----------+-----------+------------
RX RR | Send RA | | |
------------+--------------------+-----------+-----------+------------
RX RA | Adjust T3 | | Cancel T1 |
| Goto ADJ Seen RA | | Adjust T3 |
----------- +--------------------+-----------+-----------+------------
RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | |
------------+--------------------+-----------+-----------+------------
RX IIH w/o | Cancel T1 (Point- | | |
Restart TLV| to-point only) | | |
------------+--------------------+-----------+-----------+------------
T1 Expires | Send IIH/RR |Send IIH/RR|Send IIH/RR|
| Restart T1 | Restart T1| Restart T1|
------------+--------------------+-----------+-----------+------------
T1 Expires | Send IIH/ | Send IIH/ | Send IIH/ |
nth time | normal | normal | normal |
------------+--------------------+-----------+-----------+------------
T2 expires | Trigger SPF | | |
| Goto SPF Wait | | |
------------+--------------------+-----------+-----------+------------
T3 expires | Set OL | | |
| Flood local LSPs | | |
| Update fwd plane | | |
------------+--------------------+-----------+-----------+------------
LSP DB Sync| Cancel T2, and T3 | | |
| Trigger SPF | | |
| Goto SPF wait | | |
------------+--------------------+-----------+-----------+------------
All SPF | | | | Clear OL
done | | | | Update fwd
| | | | plane
| | | | Flood local
| | | | LSPs
| | | | Goto Running
======================================================================
4.3. Starting Router
Event | Starting | ADJ Seen RA| ADJ Seen CSNP
=============================================================
Router | Send IIH/SA | |
starts | Start T1,T2 | |
-------------+-------------------+------------+---------------
RX RR | Send RA | |
-------------+-------------------+------------+---------------
RX RA | Goto ADJ Seen RA | | Cancel T1
-------------+-------------------+------------+---------------
RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 |
-------------+-------------------+------------+---------------
RX IIH w | Cancel T1 | |
no Restart | (Point-to-Point | |
TLV | only) | |
-------------+-------------------+------------+---------------
ADJ UP | Start T1 | |
| Send local LSPs | |
| w OL | |
-------------+-------------------+------------+---------------
T1 Expires | Send IIH/RR |Send IIH/RR | Send IIH/RR
| and SA | and SA | and SA
| Restart T1 |Restart T1 | Restart T1
-------------+-------------------+------------+---------------
T1 Expires | Send IIH/SA |Send IIH/SA | Send IIH/SA
nth time | | |
-------------+-------------------+------------+---------------
T2 expires | Clear OL | |
| Send IIH normal | |
| Goto Running | |
-------------+-------------------+------------+---------------
LSP DB Sync | Cancel T2 | |
| Clear OL | |
| Send IIH normal | |
==============================================================
5. Security Considerations
Any new security issues raised by the procedures in this document
depend upon the ability of an attacker to inject a false but
apparently valid IIH, the ease/difficulty of which has not been
altered.
If the RR bit is set in a false IIH, neighbors who receive such an
IIH will continue to maintain an existing adjacency in the "UP" state
and may (re)send a complete set of CSNPs. While the latter action is
wasteful, neither action causes any disruption in correct protocol
operation.
If the RA bit is set in a false IIH, a (re)starting router which
receives such an IIH may falsely believe that there is a neighbor on
the corresponding interface which supports the procedures described
in this document. In the absence of receipt of a complete set of
CSNPs on that interface, this could delay the completion of (re)start
procedures by requiring the timer T1 to time out the locally defined
maximum number of retries. This behavior is the same as would occur
on a LAN where none of the (re)starting router’s neighbors support
the procedures in this document and is covered in Sections 3.3.1 and
3.3.2.
If an SA bit is set in a false IIH, this could cause suppression of
the advertisement of an IS neighbor which could either continue for
an indefinite period, or occur intermittently with the result being a
possible loss of reachability to some destinations in the network
and/or increased frequency of LSP flooding and SPF calculation.
The possibility of IS-IS PDU spoofing can be reduced by the use of
authentication as described in [1] and [2], and especially the use of
cryptographic authentication as described in [5].
6. IANA Considerations
This document defines the following IS-IS TLV that is listed in the
IS-IS TLV code-point registry:
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
211 Restart TLV y n n
7. Normative References
[1] Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
December 1990.
[2] 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, Second Edition.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Katz, D. and R. Saluja, "Three-Way Handshake for IS-IS Point-
to-Point Adjacencies", RFC 3373, September 2002.
[5] Li, T. and R. Atkinson, "Intermediate System to Intermediate
System (IS-IS) Cryptographic Authentication", RFC 3567, July
2003.
8. Acknowledgements
The authors would like to acknowledge contributions made by Jeff
Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth,
Russ White, and Rena Yang.
9. Authors’ Addresses
Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
Phone: +44 208 824 8690
EMail: mshand@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
EMail: ginsberg@cisco.com
10. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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