Internet DRAFT - draft-lu-isis-transaction-tlv
draft-lu-isis-transaction-tlv
Network Working Group W. Lu
Internet-Draft A. Tian
Intended status: Standards Track Ericsson
Expires: September 6, 2012 March 5, 2012
ISIS Transaction TLV
draft-lu-isis-transaction-tlv-00
Abstract
ISIS local updates may require multiple LSPs to convey. Receiving
routers, whose decision processes are without such knowledge, may
generate incorrect routing table updates based on the partial set of
LSPs it receives and hence the traffic outage before they are
corrected by another run of the decision process. This memo
describes a method that makes the decision process more informed so
that the interim results can be minimized or avoided.
Status of this Memo
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This Internet-Draft will expire on September 6, 2012.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Implicit Purging . . . . . . . . . . . . . . . . . . . . . 3
1.4. PDU Transaction Knowledge . . . . . . . . . . . . . . . . 4
2. Transaction TLV . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. LSP Transaction Set . . . . . . . . . . . . . . . . . . . 5
2.2. TLV Format . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. T-TLV Count . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Transaction ID . . . . . . . . . . . . . . . . . . . . . . 6
3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Originator . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Receiver . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Opening the Transaction . . . . . . . . . . . . . . . 7
3.2.2. Invalid Transaction . . . . . . . . . . . . . . . . . 8
3.2.3. Processing T-TLV . . . . . . . . . . . . . . . . . . . 8
3.2.4. Closing the Transaction . . . . . . . . . . . . . . . 8
3.2.5. Aborting the Transaction . . . . . . . . . . . . . . . 9
3.2.6. Exit Transaction . . . . . . . . . . . . . . . . . . . 9
3.2.7. Timer Expiry . . . . . . . . . . . . . . . . . . . . . 9
3.2.8. TID Wrap . . . . . . . . . . . . . . . . . . . . . . . 9
4. Multiple Transaction Sets . . . . . . . . . . . . . . . . . . 9
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Avoid Unwanted Purging . . . . . . . . . . . . . . . . . . 10
5.2. Allow reordering of TLVs in GR case . . . . . . . . . . . 10
5.3. Help Precise SPF Scheduling . . . . . . . . . . . . . . . 10
5.4. Other than SPFs . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Link state protocols run on the knowledge of the entire topology.
Incomplete topology information, even temporary, can result in
traffic outage or routing loop. While transitional routing changes
are inevitable and common to both OSPF [RFC2328] and ISIS
[RFC1195][ISO.10589.1992], impacts to unchanged network connectivity
are unnecessary and should be minimized if not totally avoidable.
1.1. Requirements Language
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 [RFC2119].
1.2. Acronyms
IS-IS - Intermediate System to Intermediate System
OSPF - Open Shortest Path First
TLV - Type Length Value
PDU - Protocol Data Unit
LSP - Link State PDU
SPF - Shortest Path First
1.3. Implicit Purging
Compared to OSPF, these impacts are unique to ISIS. There are two
reasons. One is that ISIS LSPs use implicit TLV purging. Although
LSPs do have age field which can be used for purging purpose, ISIS
does not have the age granularity down to TLV level which is the
atomic unit of ISIS link state information. If for some reason a TLV
needs to be relocated to a different LSP fragment (e.g. TLV-B in
Figure 1 and Figure 2), this TLV can be perceived as being purged
from the original LSP fragment. And if the receiving ISIS starts its
decision process before it sees the second LSP fragment, the
reachability via this TLV, if any, will be lost.
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LSP-Foo.00-00 LSP-Foo.00-01
-------------------------------- -------------------------------
| ............ | TLV-A | TLV-B | | ........... | TLV-X | ----- |
-------------------------------- -------------------------------
Figure 1: Frag 00 Almost Full
LSP-Foo.00-00 LSP-Foo.00-01
-------------------------------- -------------------------------
| ............ | TLV-A | - | | ........... | TLV-X | TLV-B |
-------------------------------- -------------------------------
Figure 2: TLV-A grow out TLV-B
The second reason is that operating directly on data link layer, ISIS
cannot extend the LSP size beyond the MTU limit as opposed to OSPF
which can leverage the IP fragmentation capability to extend its LSU
size. The consequence is that when an LSP fragment is full or nearly
full, if some of its TLVs need to expand, they will have to be
relocated to other LSP fragment. Alternatively some other TLVs can
be moved out of this LSP fragment to make room for the needy. Either
way an implicit purge condition is created.
1.4. PDU Transaction Knowledge
The above issues can be mitigated if the receiving routers are
provided with LSP transaction information. In other words, if the
receivers know how many LSPs they should expect from a particular
originating Intermediate System, so that they acquire complete
topology updates from that System, the receivers should be able to
avoid running their decision process based on the incomplete
transitional link state information.
There can be many ways to accomplish the purpose. However to be
practical the solution should meet following requirements:
1. It must be backward compatible. Adding a new TLV can easily
fulfill this;
2. The TLV should be simple and short, that it does not take
significant LSP space;
3. The solution should be fallback-able. That is, in case of errors
or mistakes, it can fallback to the operation state without such
solution;
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4. It can be implemented easily without adding much burden on the
originator and its update process. In particular it should not
delay or change the timing of LSP flooding;
5. On the receiver side, the new logic should be simple and can be
easily integrated to the existing logic, such as SPF scheduler.
Performance wise, the new addition should be negligible.
This document describes a transaction knowledge based TLV that can be
used by the receiving routers to make informed decision.
2. Transaction TLV
A new TLV is introduced to indicate that the carrying LSP needs
additional LSPs to complement. For example, in Figure 2 LSPs "00"
and "01" both have to be included in the SPF to reflect the correct
change. If the receiver kicks off its SPF right after receiving LSP
"00" and before seeing LSP "01", the reachability pertaining to TLV-B
will be incorrectly removed, cause temporary traffic loss. The TLV
is called Transaction TLV as it provides the transaction knowledge of
the changes in which a set of LSPs are involved.
2.1. LSP Transaction Set
LSPs which are coherent in contents are called LSP Transaction Set.
These LSPs must be processed atomically by the receiver's decision
process to avoid incorrect result. In Figure 2, LSPs "00" and "01"
form the LSP transaction set.
The LSP transaction set is a subset of all LSPs originated by an IS.
In other words, the transaction set belongs to a single IS, and
shares the same LSP ID which is the System ID plus the pseudo node
ID.
2.2. TLV Format
Figure 3 describes the Transaction TLV data fields:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Len | TID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count |
+-+-+-+-+-+-+-+-+
Figure 3: Transaction TLV Format
Type
1 byte, value TBD (T-TLV);
Len
1 byte, 4 or 5;
TID
4 bytes, Transaction ID;
Count
1 byte, optional.
The TLV can appear at most once in an LSP (fragment). Each LSP in
an LSP transaction set is encoded with the same TLV except the
last LSP which MUST include the "count" in this TLV to tell the
total number LSPs in this transaction set.
2.3. T-TLV Count
The count field is of 1 byte size, same as the size for LSP number
field. The first "count-1" LSPs use the shorter T-TLV which has
length 4. The last LSP use the longer T-TLV which contains the count
field which counts in itself. This design is to make the originator
easy to encode the TLV without having to know the count beforehand.
2.4. Transaction ID
Along the time, an IS may generate and flood a number of LSP
transaction sets. To differentiate one set from another, a
monotonically increased Transaction ID (TID) is used. Each IS
maintains and manages its TID. If an IS is also a DIS in one or more
of its interfaces, each pseudo node has its own TID which is
independent of the TID of the non-pseudo node, or other pseudo node.
In case of TID conflict (due to race condition in flooding, or
errors), the higher TID invalidates the lower TID. When a TID
reaches the maximum, the TID wrap mechanism is used, which is
detailed in Section 3.2.8.
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3. Operation
3.1. Originator
When an IS decides that its update process will have to use multiple
LSPs to convey some information atomically, it labels these LSPs with
the T-TLV and follows the procedures below:
1. The T-TLV should be used for SPF-sensitive changes only;
2. It starts a new TID number which is per LSP ID based. How to
choose the initial TID number is a local decision though the
natural choice would be 1. The number MUST be incremented from
the existing one, and monotonically increased ever since;
3. T-TLV is recommended to be added to the end of an LSP. The TID
is stored in the LSP set space (containing all LSP fragments), as
opposed to the individual LSP space;
4. Repeat step 3 until there is no more to pack, at which time a
T-TLV with count field is inserted to this last LSP in the
transaction set.
3.2. Receiver
Per ISIS protocol nature, if the receiver does not understand and
support the T-TLV, the TLV is silently ignored. This ensures the
backward compatibility.
Otherwise the receiving IS enters into the transaction procedure.
An IS will not engage its decision process into such procedure for
T-TLVs whose carrying LSPs are already installed in the database. In
other words, the procedure is activated only upon the receiving of
the T-TLVs whose carrying LSPs are new.
3.2.1. Opening the Transaction
When a T-TLV is received, the receiving IS enters the LSP transaction
procedure.
Type
The TID is recorded to indicate that the current TID is active;
Len
A protection timer is started to prevent the error case where the
transaction cannot close in time;
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TID
LSPs in a transaction set may not arrive in the order they are
sent. Whichever arrives the first opens the transaction;
Count
The transaction record (TID/count/status) is maintained under the
LSP set space (SystemID + PseudoNodeID).
3.2.2. Invalid Transaction
The transaction is invalid, and MUST be aborted (exit) if:
Type
TID is outdated. This occurs if a higher TID is found, or the
same TID is closed in the past transaction;
Len
more than one T-TLV is found in an LSP;
TID
more than one T-TLV with count field is found in an LSP
transaction set;
3.2.3. Processing T-TLV
When a T-TLV is received, following rules apply:
Type
Open the transaction if not yet, also increment the corresponding
local count. If the received TLV contains the count, note down
the announced count.
Len
If the local count equals the announced count, close the
transaction.
3.2.4. Closing the Transaction
If the received T-TLV causes the local count to match the announced
count:
Type
Change the current transaction TID from active to closed;
Len
Cancel the protection timer;
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Len
Exit the transaction.
Note that any T-TLV can close the transaction as long as it causes
the match of counters. Implementation should not assume that the
T-TLV with count field comes the last.
3.2.5. Aborting the Transaction
Any error condition can abort the current transaction. The handling
procedure is the same as the one in Section 3.2.4.
3.2.6. Exit Transaction
A transaction can be terminated normally (closing) or abnormally due
to error conditions.
Closing and aborting the transaction are technically the same
operation. The difference is that closing the transaction fulfills
the purpose of T-TLVs for avoiding unnecessary packet loss.
Either way after the transaction is terminated, the decision process
MUST no longer block its SPF and should start the computation
immediately or follow whatever SPF scheduling mandates.
3.2.7. Timer Expiry
The expiry of the protection timer indicates that some transaction
error has occurred. The receiving IS MUST abort the transaction.
The length of the timer is a local decision.
3.2.8. TID Wrap
When a TID reaches the maximum (0xFFFFFFFF), the originating IS will
have to refrain from using T-TLV for LSP maximum age (21 minutes
usually). The logic is similar to that of LSP sequence number
wrapping.
4. Multiple Transaction Sets
The count field is of 1 byte size, same as the size for LSP number
field. The first "count-1" LSPs use the shorter T-TLV which has
length 4. The last LSP use the longer T-TLV which contains the count
field which counts in itself. This design is to make the originator
easy to encode the TLV without having to know the count beforehand.
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5. Use Cases
The count field is of 1 byte size, same as the size for LSP number
field. The first "count-1" LSPs use the shorter T-TLV which has
length 4. The last LSP use the longer T-TLV which contains the count
field which counts in itself. This design is to make the originator
easy to encode the TLV without having to know the count beforehand.
5.1. Avoid Unwanted Purging
The unwanted purging described in Section 1.3 can be avoided using
T-TLVs. The originator can add T-TLV to LSP-Foo.00-00 and T-TLV
(count=2) to LSP-Foo.00-01. The receivers will withhold the SPF till
both LSPs are received. The missing TLV-B in the first LSP as shown
in Figure 2 will not be treated as an implicit purging, as it will be
found in the second LSP.
5.2. Allow reordering of TLVs in GR case
If an IS advertises lots of redistributed routes in its LSPs, it is
not trivial to maintain its TLV (like TLV 135) orders.
This is especially true when an IS has just gone through the graceful
restart process. Because the RIB does not necessary supply the
redistributed routes the same order in the Pre-Restart time,
reconstruct LSPs will result in LSPs with TLVs reordered.
And if the number of redistributed routes is high, they spread over
multiple LSPs. When the set of LSPs reaches other ISes, the same
issue of 5.1 can arise, even if there is no change to redistributed
routes at all.
It is not impossible for the originating IS to use sophisticated
means to keep those TLVs in their original order. Nevertheless this
issue can easily be addressed with the T-TLVs.
The restarting IS can add T-TLVs to all LSPs that are subject to TLV
reordering, and transmit them upon exit of its graceful restart
process. Thus the receiving ISes will not mistakenly purge some IS
external reachability prefixes.
5.3. Help Precise SPF Scheduling
As a link state protocol, ISIS has to two conflict goals. One is to
be fast responsive to the network changes. The other is the network
stability.
If the SPF is scheduled too swiftly, the system (and even the
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network) can melt down for some storm activities. On the other hand
if there is lot of delays, the network becomes too slow adapting
changes.
For example, if an IS receives a burst of redistributed routes from
BGP, it may send out dozens of LSPs for advertising all those routes.
The receiving ISes, upon received first several LSPs, usually start
the decision process to compute the new routing table. The routing
table is incomplete and will be soon overwritten by another SPF run.
If the number of routes (and hence LSPs) is high, most such SPF runs
will be useless and wasteful. Only the last SPF will contribute to
the final and correct routing table.
The T-TLV if used will provide guidance to the receiving ISes to run
SPF only when all LSPs are in place. This SPF is equivalent to the
final SPF mentioned above. Therefore it saves a lot of SPF runs and
network churns. What is more is that the T-TLV driven SPF can be
kick started immediately, compared to the final SPF which usually has
some amount of delay.
5.4. Other than SPFs
The T-TLV may also be used for some non-SPF related operation. For
example, the receiving ISes may choose to defer its TE database
uploading process until all LSPs that carry the TE information are
received.
6. Security Considerations
This proposal does not introduce additional issues on security
condition.
7. IANA Considerations
A new ISIS T-TLV is introduced. The type is TBD by IANA.
8. Acknowledgements
TBD
9. References
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9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[ISO.10589.1992]
International Organization for Standardization,
"Intermediate system to intermediate system intra-domain-
routing routine information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)",
ISO Standard 10589, 1992.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
Authors' Addresses
Wenhu Lu
Ericsson
300 Holger Way
San Jose, California 95134
USA
Email: Wenhu.Lu@ericsson.com
Albert Tian
Ericsson
300 Holger Way
San Jose, California 95134
USA
Email: Albert.Tian@ericsson.com
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