Internet Engineering Task Force Geng, Ed.
InternetDraft Shanxi Univ.
Intended status: Informational Zhang
Expires: November 13, 2020 Beihang Univ.
Shi
Wang
Yin
Tsinghua Univ.
May 12, 2020
Efficient Implementation Method for Loopfree Criterion
draftgengiaccaba00
Abstract
[RFC5286] introduces LoopFree Criterion (LFC) in detail, which is a
technology for local fast rerouting when network failures occur.
With LFC, alternate next hops are stored alongside with the default
next hops in a routers forwarding table, and can be immediately
activated to invoke a loop free repair path in face of link failure.
As long as not introducing routing loops, these alternative next hops
can also be used for multipath transmission if there are stringent
demands on bandwidth or load balancing. However, in such link state
networks, computing loop free alternates typically requires one or
more rounds of full shortest path tree computation on a graph, and
poses a heavy burden to both the processor load and the memory
consumption of a network equipment. In this document, we describe an
efficient Loopfree Criterion (LFC) implementation method which is
based on incremental shortest path first (iSPF), which is suitable
for practical deployment in large scale networks. The computational
complexity of the method is independent of the average node degree of
the network.
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This InternetDraft will expire on November 13, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Overview of Solution . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Existing algorithms for computing LFC rely on one or more rounds of
full shortest path tree computation on a graph, and cannot achieve
both good coverage of alternates and low computational complexity at
the same time. Based on graph properties we newly find, this
document propose Incremental Alternates Computation IAC, which can
compute the full set of alternates for a given network topology in a
highly efficient way. IAC performs incremental shortest path
computation on specific link cost update, where the sign of some cost
is simply reversed.
2. Terminology
In this document, we employ OSPF as an example to explain our method.
Each router in a single routing area maintains an identical network
map which allows them to compute the shortest path to every other
router in a routing area. Then each router construct its FIB table
employing the above information. When a packet arrives at a router,
a destination address based method is using to determine how to
forward the packets to its corresponding interface. When the network
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topology changes, the routers adjacent to the changed component
detects the change and then propagates the information to its
neighboring router through the LSA (link state advertisement)
information. After a period of time, all routers in this routing
area are aware of the change information and update their routing
tables accordingly, then the network is at a stable state.
LFC: x can be chosen when C(x,d) is lower than C(x,c)+C(c,d), which
means when packets are routed from x to d, they will not be routed
back to c, since C(x,c) + C(c,d) is the lowest cost of any path from
x to d that passes c. So the protection route will bypass c, thus
bypass link (c,b) too.
In order to implement LFC rule, each node needs to obtain costs for
its neighbors as well as itself. With a linkstate protocol, it
requires multiple shortestpath computations. In this document, the
notation C(x,d) refers to the shortest path cost from node x to node
d, x refers to the neighboring node of node c. w(c,x) refers to the
weight of link (c,x), Tc refers to the shortest path tree rooted at
node c, Tc(c,x) refers to the shortest path tree rooted at node c
when the weight of the link (c,x) is changed to w(c,x), D(Tc,x)
refers to the descendants of node x (x is included) in the Tc,
D(Tc(c,x),x) refers to the descendants of node x (x is included) in
the Tc(c,x).
3. Overview of Solution
In general, in order to compute the LFCs set for a spcific
destination d, a router needs to know the following information:
(1) the shortest path cost from the calculating router, for example
router c, to the destination d (C(c,d)).
(2) the shortest path cost from the neighboring node x of c to the
destination d (C(x,d)).
(3) the shortest path cost from the neighboring node x of c to itself
(C(x,c)).
C(c,d) can be obtained from the shortest path tree Tc, C(x,c) is
equal to w(x,c) which can be obtained from OSPF protocol, C(x,c) can
be obtained by performing additional SPF calculations. Therefore,
the induced cost will be particularly high for high degree nodes.
This document describes how to efficiently implement LFC rule on
backbone networks. In order to implement LFC efficiently, the next
hop computation rule is proposed.
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Next hop computation rule:
For a node d, if d is not in the set of D(Tc,x), while d is in the
set of D(Tc(c,x),x), we can get the node x is a valid next hop from c
to d.
c
/\
3/ \5
/ \
a b
 
3  3
 
de
3
Figure 1: Network Topology
c
/\
3/ \5
/ \
a b
 
3 3
 
d e
Figure 2: Shortest path first rooted at node c
c
/\
3/ \5
/ \
a b

3

d

3

e
Figure 3: Shortest path first rooted at node c when the weight of
link (c,a) is changed to 3
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c
/\
3/ \5
/ \
a b

3

e

3

d
Figure 4: Shortest path first rooted at node c when the weight of
link (c,b) is changed to 5
IAC cleverly uses Next hop computation rule, so it can realize LFC
efficiently. IAC is suitable for incremental deployment within a
network, including a network that is already deploying iSPF. We will
use the following example to explain how ICA works. Fig. 1 depicts a
network topology consisting of 5 nodes and 6 edges, while the
corresponding SPT Tc is depicted in Fig. 2(b), with c being the root.
Fig. 3 shows the shortest path tree constructed using iSPF when the
weight of link (c,a) is changed to 3. Fig. 4 is the shortest path
tree constructed using iSPF when the weight of link (c,b) is changed
to 5. We can see that node a can be a viable backup nexthop from c
to e according to the next hop computation rule. We can get that
node b can be a viable backup nexthop from c to d in the same way.
4. Security Considerations
The security considerations of [RFC5286] also apply.
5. Conclusions
The purpose of this document is to describe an efficient way to
implement LFC, which can compute the full set of alternates with
incremental shortest path first computation on specific link cost
update. Therefore, IAC is suitable for deploying in the larger scale
networks.
6. Normative References
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: LoopFree Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
.
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Authors' Addresses
Haijun Geng (editor)
Shanxi Univ.
Taiyuan
CN
Email: genghaijun@sxu.edu.cn
Han Zhang
Beihang Univ.
Beijing
CN
Email: zhhan@buaa.edu.cn
Xingang Shi
Tsinghua Univ.
Beijing
CN
Email: shixg@cernet.edu.cn
Zhiliang Wang
Tsinghua Univ.
Beijing
CN
Email: wzl@cernet.edu.cn
Xia Yin
Tsinghua Univ.
Beijing
CN
Email: yxia@tsinghua.edu.cn
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