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Versions: 00 01
Network Working Group Alex Zinin
Internet Draft Mike Shand
Expiration Date: September 2001 Cisco Systems
File name: draft-ietf-ospf-isis-flood-opt-01.txt March 2001
Flooding optimizations
in link-state routing protocols
draft-ietf-ospf-isis-flood-opt-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
The flooding algorithm is one of the most important parts of any link
state routing protocol. It ensures that all routers within a link
state domain converge on the same topological information within a
finite period of time. To ensure reliability, typical implementations
of the flooding algorithm send new information via all interfaces
other than the one the new piece of information was received on. This
redundancy is necessary to guarantee that flooding is performed
reliably, but implies considerable overhead of utilized bandwidth and
CPU time if neighboring routers are connected with more than one
link. This document describes a method that reduces this overhead.
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1 Introduction
In order to guarantee convergence of a link state routing protocol,
it is vital to ensure that link state PDUs (LSAs in the case of OSPF
[Ref1] or LSPs in the case of ISIS [Ref2]) that are originated after
the initial LSDB synchronization between neighbors is completed are
delivered to all routers within the flooding scope limits (an area or
the whole AS depending on the protocol and the type of the link state
PDU).
The method used by link state protocols to achieve this implies that
a) PDUs are transmitted reliably between any pair of routers, and b)
whenever a new PDU is received, it is sent across all interfaces
other than the one it was received on (except for the case when the
router is the DR in OSPF, where the LSA is sent back over the same
interfaces, see [Ref1]).
To fulfil the first requirement, link state routing protocols keep
retransmitting new PDUs to the neighbors that have not acknowledged
reception (the only exception is flooding performed on broadcast
links in ISIS, see [Ref2]). As an example, in OSPF, a link state
retransmission list is maintained for every neighbor data structure
on every interface. When an LSA is sent through an interface, it is
put on the retransmission list of every neighbor associated with this
interface and is removed from it only after the neighbor has
acknowledged reception of the LSA. Similarly, ISIS implementations
use SRMflag and SSNflag that are interface-specific, as well as
periodical CSNP announcements on broadcast links to ensure
reliability of flooding.
2 Motivation
If multiple links connect two routers, flooding of new information
will cause considerable overhead of link bandwidth and CPU time spent
by the protocol. Let's consider an example shown in Figure 1.
| |
| _____ 1 ___ |
-| +--+/ . . \+--+ |-
|---|R1| : : |R2|--|
-| +--+\_____ N ___/+--+ |-
| |
| |
Figure 1. Sample topology
When R1 receives a new PDU from its LAN segment, it installs it in
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its LSDB and submits for flooding through all of its interfaces.
Since flooding presumes sending the new PDU over all interfaces
except for the one it was received on routers end up doing the
following.
1) R1 sends not one, but N copies of the new PDU to R2.
2) Only the first copy of the PDU is actually installed
in R2's LSDB, but link bandwidth and CPU cycles are
spend to transmit and process all N copies.
3) Furthermore, when R2 receives the first copy of the LSA
and installs it, it floods back to R1 N-1 copies of it,
again spending extra bandwidth and CPU time.
4) If R1 receives an acknowledgment from R2 on some links,
but not from others, it will keep retransmitting unacknowledged
LSAs though they are already in R2's LSDB.
The solution described in this document provides a technique to
minimize the overhead that link state routing protocols cause in the
described situation and use link bandwidth more efficiently.
While the described optimization is generic for OSPF and ISIS, it
becomes very important in the contents of MPLambdaS [Ref3] where OXCs
may be connected with a large number of links. The problem is
partially addresses by LMP [Ref4] where a single control channel is
used for a bundle of optical links or lambdas. However, OXCs may
still have a big amount of control channels between each other, and
some type of OXCs may not be running LMP at all. The flooding
optimizations described in this document ensure scalability of the
IGP flooding algorithm in the presence of multiple links between
neighboring routers and increasing amount of traffic engineering
information flooded in optical and MPLS networks.
3 Proposed solution
3.1 Introduction
The main idea of the technique described in this document is to move
the flooding algorithm from the per-interface to per-neighbor basis
in a backward-compatible manner.
The technique is generic for all protocols utilizing reliable
flooding and is based on the observation that the ultimate goal of
the flooding algorithm is not to send link state PDUs over all
interfaces, but to deliver them to all routers in the network.
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To implement this optimization, it is necessary to maintain a list of
neighbors within an area. Whenever a new neighbor is discovered on an
interface belonging to the area, the corresponding interface neighbor
data structure is linked to the corresponding element in the list of
neighbors. Based on the information in the list of neighbors, as well
as on the type of interfaces they use, interfaces within the area are
marked either flooding-active or flooding-passive. The process of
election of flooding-active interfaces takes into consideration the
costs of interfaces, giving preference to faster interfaces.
Multiaccess interfaces need special treatment, since they may be
(usually are) associated with more than one neighbor. However, if
such an interface connects only two routers, it still may be marked
as flooding-passive. Whenever the number of entries in the list or
state of the adjacency in the list changes, the interface election
algorithm is rerun.
Note that since the flooding paradigm is changed from the per-
interface to per-neighbor basis, PDU retransmission is not performed
for a specific neighbor on a specific interface, but is instead done
for a specific neighbor in general, and it is enough to receive a
single acknowledgment on any interface for sending router to stop
retransmitting.
The asynchronous flooding algorithm is changed to first consider the
area neighbor list and then use available physical interfaces to
reliably deliver link state PDUs to the neighbors. Note that if more
than one interface to a particular neighbor is marked as flooding-
active, the flooding algorithm may perform equal- or unequal-cost
load sharing, flooding different PDUs through different links.
Flooding is also changed not to send PDUs to the sending neighbor via
other links.
The initial process of LSDB synchronization is also changed to take
advantage of multiple links. If a new adjacency is coming up and the
router can be sure that its LSDB is already synchronized with the
remote router over other links, the router can speed up the adjacency
establishment process by sending a empty (or limited-size) database
description to the remote neighbor. Note that this speeds up the
announcement of links that come up, since OSPF announces an adjacency
only when it reaches Full state, i.e., when routers have synchronized
their LSDBs. Skipping the LSDB synchronization part in ISIS does not
speed link announcement (since ISIS announces adjacency as soon as
two-way connectivity has been ensured), but it reduces the amount of
time CPU spends on processing of CSNPs and LSPs.
To illustrate the benefits of the described method, consider the
situation where R1 in Figure 1 has 100 PDUs to flood to R2, and N
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equals 3. Without the described optimization, we would have:
o 300 copies of PDUs going from R1 to R2
o 300 LSDB lookups performed by R2
o 300 acknowledgements coming back from R2
o 300 lookups on the retransmit list or LSDB by R1 to remove
acknowledged PDUs
o 200 copies of PDUs coming back from R2 to R1
o 200 LSDB lookups performed by R1
o 200 ackowledgments going from R1 to R2
o 200 lookups on the retransmit list or LSDB by R2 to remove
acknowledged PDUs
If described technique is implemented, we would have:
o 100 copies of PDUs going from R1 to R2 possibly over dif-
ferent interfaces for faster transmission
o 100 LSDB lookups performed by R2
o 100 acknowledgements coming back from R2
o 100 lookups on the retransmit list or LSDB by R1 to remove
acknowledged PDUs
o no copies of PDUs coming back from R2 to R1
o no LSDB lookups performed by R1
o no acknowledgments going from R1 to R2
o no lookups on the retransmit list or LSDB by R2 to remove
acknowledged PDUs
3.2 Changes to OSPF
3.2.1 Data structures
Some basic modifications to OSPF data structures are necessary to
implement the described solution.
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First of all, a new field is introduced to the area data structure,
called NeighborList. NeighborList is a list of entries, each contain-
ing the following fields. Note that the NeighborList entry is created
when the first neighbor data structure for the neighbor with a par-
ticular router ID is created within an area.
o NeighborID---the router ID of the neighbor connected with
the calculating router with one or more interfaces.
o P2pIntList---list of interfaces that have only one fully
established adjacency and it is established with the neigh-
bor identified by the NeighborID (point-to-point and virtual
links, as well as broadcast and NBMA interfaces connecting
only two routers).
o P2mpIntList---list of interfaces that have more than one
fully established adjacency and one of them is established
with the neighbor identified by the NeighborID (apparently
NBMA and broadcast networks).
o Retransmission list---list of LSAs that must be delivered to
the remote router using available physical interfaces.
Note that when a neighbor is reachable over multiple interfaces,
there will be more than one entry in the above lists of interfaces.
A new field is introduced to the interface-specific neighbor data
structure---NeighborEntry. When an instance of the interface-specific
neighbor data structure is created, its NeighborEntry is set to
reference the corresponding entry in the area neighbor list. Note
that whenever a neighbor data structure is created for an interface,
the P2pIntList and P2mpIntList of area neighbor data structures
corresponding to the neighbors reachable through the same interface
are modified. If only one neighbor data structure is available for
the interface, the interface is put on the P2pIntList for that neigh-
bor. Otherwise, if more than one neighbor is known (regardless of
the state of the neighbors), the interface is placed on the
P2mpIntLists of all neighbors reachable through that interface. Note
that point-to-point interfaces may temporarily have more than one
neighbor linked to the interface data structure when the router-ID of
the neighbor is changing. The algorithm handles such transition
states by temporarily putting the interface on the P2mpIntList.
Also, a new field is introduced to the interface data structure,
called FloodingActive. If the value of this field is TRUE, the inter-
face is used for flooding. Otherwise the interface is flooding- pas-
sive and no LSAs are sent over it when asynchronous flooding is per-
formed. Note that whenever an interface is put on a P2mpIntList of
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any area neighbor data structure, its FloodingActive field is always
set to TRUE. Actually, FloodingActive field is consulted only if the
interface is in a P2pIntList.
Another field introduced to the interface data structure is LSASent,
that is used by the flooding procedure (see Section 3.2.3 for more
details).
Whenever there is a change in the contents of the P2pIntList or
P2mpIntList of an area neighbor data structure, the router performs
election of flooding-active interfaces among the interfaces listed in
the P2pIntList field.
Below follows the algorithm describing the election process. Note
that this algorithm produces the minimal set of active interfaces.
Implementations may use different algorithms, but these algorithms
must not produce a smaller set of interfaces.
For every entry in the area neighbor list, do the following.
1. If the P2mpIntList is not empty, go through all interfaces
in the P2pIntList and mark them flooding-passive by setting
the FloodingActive interface field to FALSE. (We always
prefer sending LSAs to multiple neighbors simultaneously).
2. Otherwise, among the interfaces in the P2pIntList, set
FloodingActive field to TRUE for those interfaces that have
the best interface cost. Set it to FALSE for all other
interfaces in the list.
3.2.2 Initial LSDB synchronization
Implementations may decide to maintain a single link state request
list per neighbor in an area. This may be used to split the Loading
process among several links when more than one adjacency is coming up
simultaneously. Note that in this case, whenever the link state
request list for a particular neighbor becomes empty, a LoadingDone
event should be generated for all adjacencies with this neighbor that
are currently in the Loading state.
3.2.3 Asynchronous Flooding
Asynchronous flooding algorithm is changed as follows. Note that
changes described below do not affect flooding back to a multiaccess
interface if the router is the DR. The changes are only in the part
where LSA is sent over other interfaces.
If the flooding scope is domain-wide, perform the following for
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all areas. If the flooding scope is area-wide, do the following
steps only for the area the interface on which the LSA was
received belongs to.
Consider every neighbor element in the area neighbor list as fol-
lows.
1) If the value of the NeighborID field is equal to the router
ID of neighbor that sent the LSA to the router, consider the
next neighbor element (there is no need to send the LSA back
to the sending router, except for the case when the receiv-
ing router is the DR and the LSA is flooded back to a
mutliaccess interface).
2) If P2mpIntList is empty, go to step 3. Otherwise do the fol-
lowing steps
a) Put the LSA on the neighbor's retransmission list
b) Go through every interface on the P2mpIntList and do
the following:
o Compare the LSA being flooded and the one identi-
fied by the LSASent field of the interface data
structure. If the LSAs are the same, the LSA has
already been sent on this interfaces and next
interface in P2mpIntList must be considered.
o Send the LSA in a link state update packets set-
ting the destination address according to the
rules in Section 13 of [Ref1]
o Set LSASent field of the interface data structure
to the LSA that has just been sent.
c) Consider the next neighbor element in the area
neighbor list. (That is, skip flooding over inter-
faces in the P2pIntList.)
3) If P2pIntList is empty, consider the next neighbor element.
Otherwise:
a) Put the LSA on the neighbor's retransmission list
b) Go through every interface on the P2pIntList and do the
following:
o If the interface FloodingActive flag is clear,
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skip this interface and consider the next inter-
face in the list.
o Send the LSA in a link state update packets set-
ting the destination address according to the
rules in Section 13 of [Ref1]
c) Consider the next neighbor element in the area neighbor
list.
Reception of OSPF acknowledgements is modified as follows.
Whenever a link state acknowledgement is received from a neighbor,
the corresponding entry in the area neighbor list is located and
corresponding LSA is removed from the retransmission list.
3.2.4 Retransmitting LSAs
The OSPF implementation should also be modified to perform
retransmission of LSAs on a per-neighbor basis.
Normally, the interfaces for LSA retransmission should be selected
according to the rules used for asynchronous LSA flooding. However,
implementations may consider retransmitting LSAs over a bigger set of
interfaces leading to the neighbor if the minimal interface set is
suspected to be not sufficient (because of link load, or packet
drops) to complete LSDB synchronization within a reasonable period of
time.
3.2.5 Opaque LSA Support
Described optimizations can be applied to all LSAs, including Opaque
LSAs that have area and domain wide flooding scope (type-10 and
type-11). Note however, that transmission of type-9 LSAs (that have
link-local flooding scope) should remain intact.
3.2.6 Compatibility
The optimization described above is designed to be backward-
compatible. No software modification is necessary for the neighbor-
ing routers. However, if both routers support the described modifica-
tions, the advantages will be greater.
3.3 Changes to ISIS
The changes for IS-IS are similar to those for OSPF.
Each non-broadcast circuit has associated with it the system ID of
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the neighbor that is adjacent over that circuit. At each of level 1
and level 2, the set of one or more circuits with an adjacency at
that level and a common neighbor is identified as a group. SRMflags
are associated with groups rather than circuit. SSNflags remain
associated with circuits.
ISO/IEC 10589 describes the setting or clearing of SRMflag or SSNflag
on a non-broadcast circuit for the following reasons.
1. SSNflag is cleared after the transmission of a PSNP over cir-
cuit C.
2. A packet has been received on circuit C and a flag on circuit
C set or cleared as a result.
3. A packet has been received on circuit C and the flags on all
other circuits set or cleared as a result.
4. The flags on all circuits are set or cleared.
These actions are modified as described below. In these descriptions,
the term Sxxflag refers to either SSNflag or SRMflag.
1. SSNflag is cleared after the transmission of a PSNP over cir-
cuit C.
Clear the flag on circuit C.
2. A packet has been received on circuit C and an Sxxflag on cir-
cuit C is to be set or cleared as a result. Circuit C is a
member of group G.
a. If an SSNflag is to be cleared, clear ALL SSNflags for
circuits in group G.
b. If an SRMflag is to be cleared, clear the SRMflag for
group G.
c. If an SSNflag is to be set, set the SSNflag for circuit C
only.
d. If an SRMflag is to be set, set the SRMflag for group G.
3. A packet has been received on circuit C and the Sxxflags on
all other circuits are to be set or cleared as a result. Cir-
cuit C is a member of group G.
a. If an SSNflag is to be cleared, clear ALL SSNflags for
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all circuits belonging to groups other than G.
b. If an SRMflag is to be cleared, clear ALL SRMflags for
groups other than G.
c. If an SRMflag is to be set, set ALL SRMflags for groups
other than G.
4. The flags on all circuits are set or cleared
a. If an SSNflag is to be cleared, clear ALL SSNflags for
all circuits.
b. If an SRMflag is to be cleared, clear SRMflags for all
groups.
c. If an SRMflag is to be set, set SRMflags for all groups.
5 Transmitting an LSP as a result of SRMflags being set on group
G.
Choose ONE circuit from group G, and transmit the LSP
over that circuit.
Where a circuit is required to be chosen from within a group, the
choice made is implementation dependant and may be based on any cri-
teria, such as bandwidth or management control. The result of the
choice MAY be different on each occasion. Implementations may also
decide to choose no point-to-point links if a neighboring system is
available via a broadcast circuit, since LSPs need to be flooded
throught it anyway. It is also possible to treat broadcast circuits
with only two routers attached as point-to-point circuits (inlcuding
Hello PDUs and LSDB synchronization pocess), note however that the
routers should be explicitly configured to do so.
4 Security issues
This document does not introduce any new security issues to ISIS or
OSPF.
5 Acknowledgements
The authors would like to acknowledge Tony Przygienda, Yakov Rekhter,
and John Moy for their comments, and Jeff Learman for reviewing this
document.
6 References
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[Ref1] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
Engineering Task Force, 1998. ftp://ftp.isi.edu/in-
notes/rfc2328.txt.
[Ref2] 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.
[Ref3] D. Awduche, Y. Rekhter, J. Drake, R. Coltun,
"Multi-Protocol Lambda Switching: Combining MPLS Traffic
Engineering Control With Optical Crossconnects", draft-
awduche-mpls-te-optical-02.txt. Work in progress.
[Ref4] J. Lang, et al. "Link Management Protocol (LMP)",
draft-ietf-mpls-lmp-01.txt. Work in progress.
7 Authors' addresses
Alex Zinin Mike Shand
Cisco Systems Cisco Systems
150 West Tasman Dr. 4, The Square
San Jose,CA Stockley Park
95134 UXBRIDGE
E-mail: azinin@cisco.com Middlesex
UB11 1BN, UK
E-mail: mshand@cisco.com
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