draft-ietf-rtgwg-ipfrr-notvia-addresses-08.txt   draft-ietf-rtgwg-ipfrr-notvia-addresses-09.txt 
Network Working Group S. Bryant Network Working Group S. Bryant
Internet-Draft S. Previdi Internet-Draft S. Previdi
Intended status: Informational Cisco Systems Intended status: Informational Cisco Systems
Expires: June 23, 2012 M. Shand Expires: December 11, 2012 M. Shand
Individual Contributor Individual Contributor
December 21, 2011 June 9, 2012
IP Fast Reroute Using Not-via Addresses IP Fast Reroute Using Not-via Addresses
draft-ietf-rtgwg-ipfrr-notvia-addresses-08 draft-ietf-rtgwg-ipfrr-notvia-addresses-09
Abstract Abstract
This draft describes a mechanism that provides fast reroute in an IP This draft describes a mechanism that provides fast reroute in an IP
network through encapsulation to "not-via" addresses. A single level network through encapsulation to "not-via" addresses. A single level
of encapsulation is used. The mechanism protects unicast, multicast of encapsulation is used. The mechanism protects unicast, multicast
and LDP traffic against link, router and shared risk group failure, and LDP traffic against link, router and shared risk group failure,
regardless of network topology and metrics. regardless of network topology and metrics.
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 RFC2119 [RFC2119].
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 23, 2012. This Internet-Draft will expire on December 11, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 3 2. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 4
2.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . . 4 2.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . . 5
2.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . . 4 2.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . . 5
3. Not-via Repair Path Computation . . . . . . . . . . . . . . . 5 3. Not-via Repair Path Computation . . . . . . . . . . . . . . . 6
3.1. Computing not-via repairs in routing vector protocols . . 6 3.1. Computing not-via repairs in distance and path vector
4. Operation of Repairs . . . . . . . . . . . . . . . . . . . . . 6 routing protocols . . . . . . . . . . . . . . . . . . . . 7
4.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . . 6 4. Operation of Repairs . . . . . . . . . . . . . . . . . . . . . 7
4.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Loop Prevention Under Node Failure . . . . . . . . . . 7 4.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 7 4.2.1. Loop Prevention Under Node Failure . . . . . . . . . . 8
4.4. Installation of Repair Paths . . . . . . . . . . . . . . . 9 4.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9
5. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 10 4.4. Installation of Repair Paths . . . . . . . . . . . . . . . 10
5.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 10 5. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. Use of LFAs with SRLGs . . . . . . . . . . . . . . . . 14 5.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 11
5.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 14 5.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 16
5.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 15 5.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 17
5.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 16 5.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 17
5.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . . 17 5.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . . 18
5.3. Multiple Independent Failures . . . . . . . . . . . . . . 17 5.3. Multiple Independent Failures . . . . . . . . . . . . . . 18
5.3.1. Looping Repairs . . . . . . . . . . . . . . . . . . . 18 5.3.1. Looping Repairs . . . . . . . . . . . . . . . . . . . 19
5.3.2. Outline Solution . . . . . . . . . . . . . . . . . . . 19 5.3.2. Outline Solution . . . . . . . . . . . . . . . . . . . 20
5.3.3. Looping Repairs . . . . . . . . . . . . . . . . . . . 20 5.3.3. Looping Repairs . . . . . . . . . . . . . . . . . . . 21
5.3.3.1. Dropping Looping Packets . . . . . . . . . . . . . 20 5.3.3.1. Dropping Looping Packets . . . . . . . . . . . . . 21
5.3.3.2. Computing non-looping Repairs of Repairs . . . . . 21 5.3.3.2. Computing non-looping Repairs of Repairs . . . . . 22
5.3.4. Mixing LFAs and Not-via . . . . . . . . . . . . . . . 23 5.3.4. Mixing LFAs and Not-via . . . . . . . . . . . . . . . 24
6. Optimizing not-via computations using LFAs . . . . . . . . . . 24 6. Optimizing not-via computations using LFAs . . . . . . . . . . 25
7. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 25 8. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 26
9. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 25 9. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 26 10. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 27
11. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 26 11. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 27
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 12. Manageability Considerations . . . . . . . . . . . . . . . . . 27
13. Security Considerations . . . . . . . . . . . . . . . . . . . 26 12.1. Pre-failure configuration . . . . . . . . . . . . . . . . 28
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 12.2. Pre-failure Monitoring and operational support . . . . . . 28
15. Informative References . . . . . . . . . . . . . . . . . . . . 27 12.3. Failure action monitoring . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
14. Security Considerations . . . . . . . . . . . . . . . . . . . 29
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
16.1. Normative References . . . . . . . . . . . . . . . . . . . 30
16.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Q-Space . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction 1. Introduction
When a link or a router fails, only the neighbors of the failure are When a link or a router fails, only the neighbors of the failure are
initially aware that the failure has occurred. In a network initially aware that the failure has occurred. In a network
operating IP fast reroute [RFC5714], the routers that are the operating IP fast reroute [RFC5714], the routers that are the
neighbors of the failure repair the failure. These repairing routers neighbors of the failure repair the failure. These repairing routers
have to steer packets to their destinations despite the fact that have to steer packets to their destinations despite the fact that
most other routers in the network are unaware of the nature and most other routers in the network are unaware of the nature and
location of the failure. location of the failure.
skipping to change at page 3, line 27 skipping to change at page 4, line 27
repaired packet round the failure. The extent to which this repaired packet round the failure. The extent to which this
limitation affects the repair coverage is topology dependent. The limitation affects the repair coverage is topology dependent. The
mechanism proposed here is to encapsulate the packet to an address mechanism proposed here is to encapsulate the packet to an address
that explicitly identifies the network component that the repair must that explicitly identifies the network component that the repair must
avoid. This produces a repair mechanism, which, provided the network avoid. This produces a repair mechanism, which, provided the network
is not partitioned by the failure, will always achieve a repair. is not partitioned by the failure, will always achieve a repair.
2. Overview of Not-via Repairs 2. Overview of Not-via Repairs
This section provides a brief overview of the not-via method of This section provides a brief overview of the not-via method of
IPFRR. IPFRR. Consider the network fragment shown in Figure 1 below, in
which S has a packet for some destination D that it would normally
send via P and B, and that S suspects that P has failed.
A A
| Bp is the address to use to get | Bp is the address to use to get
| a packet to B not-via P | a packet to B not-via P
| |
S----------P----------B. . . . . . . . . .D S----------P----------B. . . . . . . . . .D
\ | Bp^ \ | Bp^
\ | | \ | |
\ | | \ | |
\ C | \ C |
\ | \ |
----------------+ X-------Y-------Z
Repair to Bp Repair to Bp
Figure 1: Not-via repair of router failure Figure 1: Not-via repair of router failure
Assume that S has a packet for some destination D that it would In the not-via IPFRR method, S encapsulates the packet to Bp, where
normally send via P and B, and that S suspects that P has failed. S Bp is an address on node B that has the property that it is not
encapsulates the packet to Bp. The path from S to Bp is the shortest reachable from node P, i.e. the notation Bp means "an address of node
path from S to B not going via P. If the network contains a path from B that is only reachable not via node P. We later show how to install
S to B that does not transit router P, i.e. the network is not the path from S to Bp such that it is the shortest path from S to B
partitioned by the failure of P, then the packet will be successfully not going via P. If the network contains a path from S to B that does
delivered to B. When the packet addressed to Bp arrives at B, B not transit router P, i.e. the network is not partitioned by the
removes the encapsulation and forwards the repaired packet towards failure of P and the path from S to Bp has been installed, then the
its final destination. packet will be successfully delivered to B. In the example we are
considering this is the path S-X-Y-Z-B. When the packet addressed to
Bp arrives at B, B removes the encapsulation and forwards the
repaired packet towards its final destination.
Note that if the path from B to the final destination includes one or Note that if the path from B to the final destination includes one or
more nodes that are included in the repair path, a packet may back more nodes that are included in the repair path, a packet MAY back
track after the encapsulation is removed. However, because the track after the encapsulation is removed. However, because the
decapsulating router is always closer to the packet destination than decapsulating router is always closer to the packet destination than
the encapsulating router, the packet will not loop. the encapsulating router, the packet will not loop.
For complete protection, all of P's neighbors will require a not-via For complete protection, all of P's neighbors will require a not-via
address that allows traffic to be directed to them without traversing address that allows traffic to be directed to them without traversing
P. This is shown in Figure 2. P. This is shown in Figure 2. Similarly, P will require a set of
not-via address (one for each neighbor) allowing traffic to be
directed to P without traversing each of those neighbors.
The not-via addresses are advertised in the routing protocol in a way
that clearly identifies them as not-via addresses and not 'ordinary'
addresses.
A A
|Ap |Ap
| |
Sp Pa|Pb Sp Pa|Pb
S----------P----------B S----------P----------B
Ps|Pc Bp Ps|Pc Bp
| |
Cp| Cp|
C C
Figure 2: The set of Not-via P Addresses Figure 2: The set of Not-via P Addresses
2.1. Use of Equal Cost Multi-Path 2.1. Use of Equal Cost Multi-Path
A router can use an equal cost multi-path (ECMP) repair in place of a A router can use an equal cost multi-path (ECMP) repair in place of a
not-via repair. not-via repair.
A router computing a not-via repair path may subject the repair to A router computing a not-via repair path MAY subject the repair to
ECMP. ECMP.
2.2. Use of LFA repairs 2.2. Use of LFA repairs
The not-via approach provides complete repair coverage and therefore The not-via approach provides complete repair coverage and therefore
may be used as the sole repair mechanism. There are, however, MAY be used as the sole repair mechanism. There are, however,
advantages in using not-via in combination with loop free alternates advantages in using not-via in combination with loop free alternates
(LFA) and or downstream paths as documented in [RFC5286]. (LFA) and or downstream paths as documented in [RFC5286]. In
particular LFAs do not require the assignment and management of
additional IP addresses to nodes, they do not require nodes in the
network to be upgraded in order to calculate not-via repair paths,
and they do not require the use of encapsulation.
LFAs are computed on a per destination basis and in general, only a LFAs are computed on a per destination basis and in general, only a
subset of the destinations requiring repair will have a suitable LFA subset of the destinations requiring repair will have a suitable LFA
repair. In this case, those destinations which are repairable by repair. In this case, those destinations which are repairable by
LFAs are so repaired and the remainder of the destinations are LFAs are so repaired and the remainder of the destinations are
repaired using the not-via encapsulation. This has the advantage of repaired using the not-via encapsulation. On the other hand, the
reducing the volume of traffic that requires encapsulation. On the path taken by an LFA repair may be less optimal than that of the
other hand, the path taken by an LFA repair may be less optimal than equivalent not-via repair for traffic destined to nodes close to the
that of the equivalent not-via repair for traffic destined to nodes far end of the failure, but may be more optimal for some other
close to the far end of the failure, but may be more optimal for some traffic. The description in this document assumes that LFAs will be
other traffic. The description in this document assumes that LFAs used where available, but the distribution of repairs between the two
will be used where available, but the distribution of repairs between mechanisms is a local implementation choice.
the two mechanisms is a local implementation choice.
3. Not-via Repair Path Computation 3. Not-via Repair Path Computation
The not-via repair mechanism requires that all routers on the path The not-via repair mechanism requires that all routers on the path
from S to B (Figure 1) have a route to Bp. They can calculate this from S to B (Figure 1) have a route to Bp. They can calculate this
by failing node P, running a Shortest Path First Algorithm (SPF), and by failing node P, running a Shortest Path First Algorithm (SPF), and
finding the shortest route to B. finding the shortest route to B.
A router has no simple way of knowing whether it is on the shortest A router has no simple way of knowing whether it is on the shortest
path for any particular repair. It is therefore necessary for every path for any particular repair. It is therefore necessary for every
router to calculate the path it would use in the event of any router to calculate the path it would use in the event of any
possible router failure. Each router therefore "fails" every router possible router failure. Each router therefore "fails" every router
in the network, one at a time, and calculates its own best route to in the network, one at a time, and calculates its own best route to
each of the neighbors of that router. In other words, with reference each of the neighbors of that router. In other words, with reference
to Figure 1, some router X will consider each router in turn to be P, to Figure 1, routers A, B, C, X, Y, Z and P will consider each router
fail P, and then calculate its own route to each of the not-via P in turn, assume that router has failed, and then calculate its own
addresses advertised by the neighbors of P. i.e. X calculates its route to each of the not-via addresses advertised by the neighbors of
route to Sp, Ap, Bp, and Cp, in each case, not via P. that router. In other words in the case of a presumed failure of P,
ALL routers (in this case S, A, B, C, X, Y and Z) calculate their
routes to Sp, Ap, Bp, and Cp, in each case, not via P.
To calculate the repair paths a router has to calculate n-1 SPFs To calculate the repair paths a router has to calculate n-1 SPFs
where n is the number of routers in the network. This is expensive where n is the number of routers in the network. This is expensive
to compute. However, the problem is amenable to a solution in which to compute. However, the problem is amenable to a solution in which
each router (X) proceeds as follows. X first calculates the base each router (X) proceeds as follows. X first calculates the base
topology with all routers functional and determines its normal path topology with all routers functional and determines its normal path
to all not-via addresses. This can be performed as part of the to all not-via addresses. This can be performed as part of the
normal SPF computation. For each router P in the topology, X then normal SPF computation. For each router P in the topology, X then
performs the following actions:- performs the following actions:-
1. Removes router P from the topology. 1. Removes router P from the topology.
2. Performs an incremental SPF (iSPF) [ISPF] on the modified 2. Performs an incremental SPF (iSPF) [ISPF] on the modified
topology. The iSPF process involves detaching the sub-tree topology. The iSPF process involves detaching the sub-tree
affected by the removal of router P, and then re-attaching the affected by the removal of router P, and then re-attaching the
detached nodes. However, it is not necessary to run the iSPF to detached nodes. However, it is not necessary to run the iSPF to
completion. It is sufficient to run the iSPF up to the point completion. It is sufficient to run the iSPF up to the point
where all of the nodes advertising not-via P addresses have been where all of the nodes advertising not-via P addresses have been
re-attached to the SPT, and then terminate it. re-attached to the SPT, and then terminate it.
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This algorithm is significantly less expensive than a set of full This algorithm is significantly less expensive than a set of full
SPFs. Thus, although a router has to calculate the repair paths for SPFs. Thus, although a router has to calculate the repair paths for
n-1 failures, the computational effort is much less than n-1 SPFs. n-1 failures, the computational effort is much less than n-1 SPFs.
Experiments on a selection of real world network topologies with Experiments on a selection of real world network topologies with
between 40 and 400 nodes suggest that the worst-case computational between 40 and 400 nodes suggest that the worst-case computational
complexity using the above optimizations is equivalent to performing complexity using the above optimizations is equivalent to performing
between 5 and 13 full SPFs. Further optimizations are described in between 5 and 13 full SPFs. Further optimizations are described in
section 6. section 6.
3.1. Computing not-via repairs in routing vector protocols 3.1. Computing not-via repairs in distance and path vector routing
protocols
While this document focuses on link state routing protocols, it is While this document focuses on link state routing protocols, it is
equally possible to compute not-via repairs in distance vector (e.g. equally possible to compute not-via repairs in distance vector (e.g.
RIP) or path vector (e.g. BGP) routing protocols. This can be RIP) or path vector (e.g. BGP) routing protocols. This can be
achieved with very little protocol modification by advertising the achieved with very little protocol modification by advertising the
not-via address in the normal way, but ensuring that the information not-via address in the normal way, but ensuring that the information
about a not-via address Ps is not propagated through the node S. In about a not-via address Ps is not propagated through the node S. In
the case of link protection this simply means that the advertisement the case of link protection this simply means that the advertisement
from P to S is suppressed, with the result that S and all other nodes from P to S is suppressed, with the result that S and all other nodes
compute a route to Ps which doesn't traverse S, as required. compute a route to Ps which doesn't traverse S, as required.
In the case of node protection, where P is the protected node, and N In the case of node protection, where P is the protected node, and N
is some neighbor, the advertisement of Np must be suppressed not only is some neighbor, the advertisement of Np MUST be suppressed not only
across the link N->P, but also across any link to P. The simplest way across the link N->P, but also across any link to P. The simplest way
of achieving this is for P itself to perform the suppression of any of achieving this is for P itself to perform the suppression of any
address of the form Xp. address of the form Xp.
4. Operation of Repairs 4. Operation of Repairs
This section explains the basic operation of the not-via repair of This section explains the basic operation of the not-via repair of
node and link failure. node and link failure.
4.1. Node Failure 4.1. Node Failure
When router P fails (Figure 2) S encapsulates any packet that it When router P fails (Figure 2) S encapsulates any packet that it
would send to B via P to Bp, and then sends the encapsulated packet would send to B via P to Bp, and then sends the encapsulated packet
on the shortest path to Bp. S follows the same procedure for routers on the shortest path to Bp. S follows the same procedure for routers
A and C in Figure 2. The packet is decapsulated at the repair target A and C in Figure 2. The packet is decapsulated at the repair target
(A, B or C) and then forwarded normally to its destination. The (A, B or C) and then forwarded normally to its destination. The
repair target can be determined as part of the normal SPF by repair target can be determined as part of the normal SPF by
recording the "next-next-hop" for each destination in addition to the recording the "next-next-hop" for each destination in addition to the
normal next-hop. normal next-hop. The next-next hop is the router that the next hop
router regards as its own next hop to the destination. In Figure 1,
B is S's next next hop to D.
Notice that with this technique only one level of encapsulation is Notice that with this technique only one level of encapsulation is
needed, and that it is possible to repair ANY failure regardless of needed, and that it is possible to repair ANY failure regardless of
link metrics and any asymmetry that may be present in the network. link metrics and any asymmetry that may be present in the network.
The only exception to this is where the failure was a single point of The only exception to this is where the failure was a single point of
failure that partitioned the network, in which case ANY repair is failure that partitioned the network, in which case ANY repair is
clearly impossible. clearly impossible.
4.2. Link Failure 4.2. Link Failure
The normal mode of operation of the network would be to assume router The normal mode of operation of the network would be to assume router
failure. However, where some destinations are only reachable through failure. However, where some destinations are only reachable through
the failed router, it is desirable that an attempt be made to repair the failed router, it is desirable that an attempt be made to repair
to those destinations by assuming that only a link failure has to those destinations by assuming that only a link failure has
occurred. occurred.
To perform a link repair, S encapsulates to Ps (i.e. it instructs the To perform a link repair, S encapsulates to Ps (i.e. it instructs the
network to deliver the packet to P not-via S). All of the neighbors network to deliver the packet to P not-via S). All of the neighbors
of S will have calculated a path to Ps in case S itself had failed. of S will have calculated a path to Ps in case S itself had failed.
S could therefore give the packet to any of its neighbors (except, of S could therefore give the packet to any of its neighbors (except, of
course, P). However, S should preferably send the encapsulated course, P). However, S SHOULD send the encapsulated packet on the
packet on the shortest available path to P. This path is calculated shortest available path to P. This path is calculated by running an
by running an SPF with the link SP failed. Note that this may again SPF with the link SP failed. Note that this may again be an
be an incremental calculation, which can terminate when address Ps incremental calculation, which can terminate when address Ps has been
has been reattached. reattached.
4.2.1. Loop Prevention Under Node Failure 4.2.1. Loop Prevention Under Node Failure
It is necessary to consider the behavior of IPFRR solutions when a It is necessary to consider the behavior of IPFRR solutions when a
link repair is attempted in the presence of node failure. In its link repair is attempted in the presence of node failure. In its
simplest form the not-via IPFRR solution prevents the formation of simplest form, the not-via IPFRR solution prevents the formation of
loops forming as a result of mutual repair, by never providing a loops as a result of mutual repair, by never providing a repair path
repair path for a not-via address. The repair of packets with not- for a not-via address. The repair of packets with not-via addresses
via addresses is considered in more detail in Section 5.3. Referring is considered in more detail in Section 5.3. Referring to Figure 2,
to Figure 2, if A was the neighbor of P that was on the link repair if A was the neighbor of P that was on the link repair path from S to
path from S to P, and P itself had failed, the repaired packet from S P, and P itself had failed, the repaired packet from S would arrive
would arrive at A encapsulated to Ps. A would have detected that the at A encapsulated to Ps. A would have detected that the AP link had
AP link had failed and would normally attempt to repair the packet. failed and would normally attempt to repair the packet. However, no
However, no repair path is provided for any not-via address, and so A repair path is provided for any not-via address, and so A would be
would be forced to drop the packet, thus preventing the formation of forced to drop the packet, thus preventing the formation of a loop.
loop.
4.3. Multi-homed Prefixes 4.3. Multi-homed Prefixes
A multi-homed Prefix (MHP) is a prefix that is reachable via more A multi-homed Prefix (MHP) is a prefix that is reachable via more
than one router in the network. Some of these may be repairable than one router in the network. Some of these may be repairable
using LFAs as described in [RFC5286]. Only those without such a using LFAs as described in [RFC5286]. Only those without such a
repair need be considered here. repair need be considered here.
When IPFRR router S (Figure 3) discovers that P has failed, it needs When IPFRR router S (Figure 3) discovers that P has failed, it needs
to send packets addressed to the MHP X, which is normally reachable to send packets addressed to the MHP X, which is normally reachable
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| | | | | |
| Sp |Pb | | Sp |Pb |
Z...............S----------P----------B...............Y Z...............S----------P----------B...............Y
Ps|Pc Bp Ps|Pc Bp
| |
Cp| Cp|
C C
Figure 3: Multi-homed Prefixes Figure 3: Multi-homed Prefixes
S should choose the closest router that can reach X during the S SHOULD choose the closest router that can reach X during the
failure as the alternate router. S determines which router to use as failure as the alternate router. S determines which router to use as
the alternate while running the SPF with P failed. This is the alternate while running the SPF with P failed. This is
accomplished by the normal process of re-attaching a leaf node to the accomplished by the normal process of re-attaching a leaf node to the
core topology (this is sometimes known as a "partial SPF"). core topology (this is sometimes known as a "partial SPF").
First, consider the case where the shortest alternate path to X is First, consider the case where the shortest alternate path to X is
via Z. S can reach Z without using the failed router P. However, S via Z. S can reach Z without using the failed router P. However, S
cannot just send the packet towards Z, because the other routers in cannot just send the packet towards Z, because the other routers in
the network will not be aware of the failure of P, and may loop the the network will not be aware of the failure of P, and may loop the
packet back to S. S therefore encapsulates the packet to Z (using a packet back to S. S therefore encapsulates the packet to Z (using a
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for MHP X. The situation now reverts to the previous case, in which for MHP X. The situation now reverts to the previous case, in which
the shortest alternate path does not require traversal of the the shortest alternate path does not require traversal of the
failure. B therefore follows the algorithm above and encapsulates failure. B therefore follows the algorithm above and encapsulates
the packet to Y (using a normal address for Y). Y removes the the packet to Y (using a normal address for Y). Y removes the
encapsulation and forwards the packet to X. encapsulation and forwards the packet to X.
It may be that the cost of reaching X using local delivery from the It may be that the cost of reaching X using local delivery from the
alternate router (i.e. Z or Y) is greater than the cost of reaching alternate router (i.e. Z or Y) is greater than the cost of reaching
X via P. Under those circumstances, the alternate router would X via P. Under those circumstances, the alternate router would
normally forward to X via P, which would cause the IPFRR repair to normally forward to X via P, which would cause the IPFRR repair to
loop. To prevent the repair from looping the alternate router must loop. To prevent the repair from looping the alternate router MUST
locally deliver a packet received via a repair encapsulation. This locally deliver a packet received via a repair encapsulation. This
may be specified by using a special address with the above semantics. may be specified by using a special address with the above semantics.
Note that only one such address is required per node. Notice that Note that only one such address is required per node. Notice that
using the not-via approach, only one level of encapsulation was using the not-via approach, only one level of encapsulation was
needed to repair MHPs to the alternate router. needed to repair MHPs to the alternate router.
4.4. Installation of Repair Paths 4.4. Installation of Repair Paths
The following algorithm is used by node S (Figure 3) to pre- The following algorithm is used by node S (Figure 3) to pre-
calculate and install repair paths in the FIB, ready for immediate calculate and install repair paths in the FIB, ready for immediate
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B. For the subset of prefixes (M) that remain (having attachment B. For the subset of prefixes (M) that remain (having attachment
point Y), install the repair path previously installed for point Y), install the repair path previously installed for
destination Y. destination Y.
For each destination (DS) that remains, install a not-via repair For each destination (DS) that remains, install a not-via repair
to Ps (P not via S). Note, these are destinations for which node to Ps (P not via S). Note, these are destinations for which node
P is a single point of failure, and can only be repaired by P is a single point of failure, and can only be repaired by
assuming that the apparent failure of node P was simply a failure assuming that the apparent failure of node P was simply a failure
of the S-P link. Note that, if available, a downstream path to P of the S-P link. Note that, if available, a downstream path to P
may be used for such a repair. This cannot generate a persistent MAY be used for such a repair. This cannot generate a persistent
loop in the event of the failure of node P, but if one neighbor loop in the event of the failure of node P, but if one neighbor
of P uses a not-via repair and another uses a downstream path, it of P uses a not-via repair and another uses a downstream path, it
is possible for a packet sent on the downstream path to be is possible for a packet sent on the downstream path to be
returned to the sending node inside a not-via encapsulation. returned to the sending node inside a not-via encapsulation.
Since packets destined to not-via addresses are not repaired, the Since packets destined to not-via addresses are not repaired, the
packet will be dropped after executing a single turn loop. packet will be dropped after executing a single turn loop.
5. Compound Failures 5. Compound Failures
The following types of failures involve more than one component: The following types of failures involve more than one component:
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3. Multiple Independent Failures 3. Multiple Independent Failures
The considerations that apply in each of the above situations are The considerations that apply in each of the above situations are
described in the following sections. described in the following sections.
5.1. Shared Risk Link Groups 5.1. Shared Risk Link Groups
A Shared Risk Link Group (SRLG) is a set of links whose failure can A Shared Risk Link Group (SRLG) is a set of links whose failure can
be caused by a single action such as a conduit cut or line card be caused by a single action such as a conduit cut or line card
failure. When repairing the failure of a link that is a member of an failure. When repairing the failure of a link that is a member of an
SRLG, it must be assumed that all the other links that are also SRLG, it MUST be assumed that all the other links that are also
members of the SRLG have also failed. Consequently, any repair path members of the SRLG have also failed. Consequently, any repair path
must be computed to avoid not just the adjacent link, but also all MUST be computed to avoid not just the adjacent link, but also all
the links which are members of the same SRLG. the links which are members of the same SRLG.
In Figure 4 below, the links S-P and A-B are both members of SRLG In Figure 4 below, the links S-P and A-B are both members of SRLG
"a". The semantics of the not-via address Ps changes from simply "P "a". The semantics of the not-via address Ps changes from simply "P
not-via the link S-P" to be "P not-via the link S-P or any other link not-via the link S-P" to be "P not-via the link S-P or any other link
with which S-P shares an SRLG" In Figure 4 this is the links that are with which S-P shares an SRLG" In Figure 4 this is the links that are
members of SRLG "a". I.e. links S-P and A-B. Since the information members of SRLG "a". I.e. links S-P and A-B. Since the information
about SRLG membership of all links is available in the Link State about SRLG membership of all links is available in the Link State
Database, all nodes computing routes to the not-via address Ps can Database, all nodes computing routes to the not-via address Ps can
infer these semantics, and perform the computation by failing all the infer these semantics, and perform the computation by failing all the
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While the use of multiple repairs in series introduces some While the use of multiple repairs in series introduces some
additional overhead, these semantics avoid the potential additional overhead, these semantics avoid the potential
combinatorial explosion of not-via addresses that could otherwise combinatorial explosion of not-via addresses that could otherwise
occur. occur.
Note that although multiple repairs are used, only a single level of Note that although multiple repairs are used, only a single level of
encapsulation is required. This is because the first repair packet encapsulation is required. This is because the first repair packet
is decapsulated before the packet is re-encapsulated using the not- is decapsulated before the packet is re-encapsulated using the not-
via address corresponding to the far side of the next link which is a via address corresponding to the far side of the next link which is a
member of the same SRLG. In some cases the de-capsulation and re- member of the same SRLG. In some cases the decapsulation and re-
encapsulation takes place (at least notionally) at a single node, encapsulation takes place (at least notionally) at a single node,
while in other cases, these functions may be performed by different while in other cases, these functions may be performed by different
nodes. This scenario is illustrated in Figure 6 below. nodes. This scenario is illustrated in Figure 6 below.
a Ps a Dg a Ps a Dg
S----------P---------G--------D S----------P---------G--------D
| | | | | | | |
| a | | | | a | | |
A----------B | | A----------B | |
| | | | | | | |
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| | | | | | | |
| b | | b | | b | | b |
C----------E---------F--------H C----------E---------F--------H
| | | |
| | | |
J----------K J----------K
Figure 7: Multiple Shared Risk Link Groups Figure 7: Multiple Shared Risk Link Groups
The link SP is a member of SRLGs "a" and "b". When a failure of the The link SP is a member of SRLGs "a" and "b". When a failure of the
link SP is detected, it must be assumed that BOTH SRLGs have failed. link SP is detected, it MUST be assumed that BOTH SRLGs have failed.
Therefore the not-via path to Ps must be computed by failing all Therefore the not-via path to Ps must be computed by failing all
links which are members of SRLG "a" or SRLG "b". I.e. the semantics links which are members of SRLG "a" or SRLG "b". I.e. the semantics
of Ps is now "P not-via any links which are members of any of the of Ps is now "P not-via any links which are members of any of the
SRLGs of which link SP is a member". This is illustrated in Figure 8 SRLGs of which link SP is a member". This is illustrated in Figure 8
below. below.
ab Ps a Dg ab Ps a Dg
S----/-----P---------G---/----D S----/-----P---------G---/----D
| | | | | | | |
| a | | | | a | | |
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C----/-----E---------F---/----H C----/-----E---------F---/----H
| | | |
| | | |
J----------K J----------K
Figure 8: Topology used for repair computation for link S-P Figure 8: Topology used for repair computation for link S-P
In this case, the repair path to Ps will be S-A-C-J-K-E-B-P. It may In this case, the repair path to Ps will be S-A-C-J-K-E-B-P. It may
appear that there is no path to D because GD is a member of SRLG "a" appear that there is no path to D because GD is a member of SRLG "a"
and FH is a member of SRLG "b". This is true if BOTH SRLGs "a" and and FH is a member of SRLG "b". This is true if BOTH SRLGs "a" and
"b" have in fact failed. But that would be an instance of multiple "b" have in fact failed, which would be an instance of multiple
uncorrelated failures which are out of scope for this design. In independent failures. In practice, it is likely that there is only a
practice it is likely that there is only a single failure, i.e. single failure, i.e. either SRLG "a" or SRLG "b" has failed, but not
either SRLG "a" or SRLG "b" has failed, but not both. These two both. These two possibilities are indistinguishable from the point
possibilities are indistinguishable from the point of view of the of view of the repairing router S and so it MUST repair on the
repairing router S and so it must repair on the assumption that both assumption that both are unavailable. However, each link repair is
are unavailable. However, each link repair is considered considered independently. The repair to Ps delivers the packet to P
independently. The repair to Ps delivers the packet to P which then which then forwards the packet to G. When the packet arrives at G, if
forwards the packet to G. When the packet arrives at G, if SRLG "a" SRLG "a" has failed it will be repaired around the path G-F-H-D.
has failed it will be repaired around the path G-F-H-D. This is
illustrated in Figure 9 below. If, on the other hand, SRLG "b" has This is illustrated in Figure 9 below. If, on the other hand, SRLG
failed, link GD will still be available. In this case the packet "b" has failed, link GD will still be available. In this case the
will be delivered as normal across the link GD. packet will be delivered as normal across the link GD.
ab Ps a Dg ab Ps a Dg
S----/-----P---------G---/----D S----/-----P---------G---/----D
| | | | | | | |
| a | | | | a | | |
A----/-----B | | A----/-----B | |
| | | | | | | |
| b | | b | | b | | b |
C----------E---------F--------H C----------E---------F--------H
| | | |
| | | |
J----------K J----------K
Figure 9: Topology used for repair computation for link G-D Figure 9: Topology used for repair computation for link G-D
If both SRLG a and SRLG b had failed, the packet would be repaired as
far as P by S, and would be forwarded by P to G. G would encapsulate
the packet to D using the not-via address Dg and forward it to F. F
would recognise that the its next hop to Dg (H) was unreachable due
to the failure of link FH (part of SRLG b) and would drop the packet,
because packets addressed to a not-via address are not repaired in
basic not-via IPFRR.
The repair of multiple independent failures is not provided by the
basic not-via IPFRR method described so far in this memo.
A repair strategy that assumes the worst-case failure for each link A repair strategy that assumes the worst-case failure for each link
can often result in longer repair paths than necessary. In cases can often result in longer repair paths than necessary. In cases
where only a single link fails, rather than the full SRLG, this where only a single link fails, rather than the full SRLG, this
strategy may occasionally fail to identify a repair even though a strategy may occasionally fail to identify a repair even though a
viable repair path exists in the network. The use of sub-optimal viable repair path exists in the network. The use of sub-optimal
repair paths is an inevitable consequence of this compromise repair paths is an inevitable consequence of this compromise
approach. The failure to identify any repair is a serious approach. The failure to identify any repair is a serious
deficiency, but is a rare occurrence in a robustly designed network. deficiency, but is a rare occurrence in a robustly designed network.
This problem can be addressed by:- This problem can be addressed by:-
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3. Using some form of SRLG diagnostic (for example, by running BFD 3. Using some form of SRLG diagnostic (for example, by running BFD
over alternate repair paths) to determine which SRLG member(s) over alternate repair paths) to determine which SRLG member(s)
has actually failed and using this information to select an has actually failed and using this information to select an
appropriate pre-computed repair path. However, aside from the appropriate pre-computed repair path. However, aside from the
complexity of performing the diagnostics, this requires multiple complexity of performing the diagnostics, this requires multiple
not-via addresses per interface, which has poor scaling not-via addresses per interface, which has poor scaling
properties. properties.
4. Using the mechanism described in Section 5.3 4. Using the mechanism described in Section 5.3
5.1.1. Use of LFAs with SRLGs
Section 4.1 above describes the repair of links which are members of
one or more SRLGs. LFAs can be used for the repair of such links
provided that any other link with which S-P shares an SRLG is avoided
when computing the LFA. This is described for the simple case of
"local-SRLGs" in [RFC5286].
5.2. Local Area Networks 5.2. Local Area Networks
LANs are a special type of SRLG and are solved using the SRLG LANs are a special type of SRLG and are solved using the SRLG
mechanisms outlined above. With all SRLGs there is a trade-off mechanisms outlined above. With all SRLGs there is a trade-off
between the sophistication of the fault detection and the size of the between the sophistication of the fault detection and the size of the
SRLG. Protecting against link failure of the LAN link(s) is SRLG. Protecting against link failure of the LAN link(s) is
relatively straightforward, but as with all fast reroute mechanisms, relatively straightforward, but as with all fast reroute mechanisms,
the problem becomes more complex when it is desired to protect the problem becomes more complex when it is desired to protect
against the possibility of failure of the nodes attached to the LAN against the possibility of failure of the nodes attached to the LAN
as well as the LAN itself. as well as the LAN itself.
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A--------S-------(N)-------------P------B A--------S-------(N)-------------P------B
| |
| |
| |
+--------------R------D +--------------R------D
Figure 10: Local Area Networks Figure 10: Local Area Networks
Consider the LAN shown in Figure 10. For connectivity purposes, we Consider the LAN shown in Figure 10. For connectivity purposes, we
consider that the LAN is represented by the pseudonode (N). To consider that the LAN is represented by the pseudonode (N). To
provide IPFRR protection, S must run a connectivity check to each of provide IPFRR protection, S MUST run a connectivity check to each of
its protected LAN adjacencies P, Q, and R, using, for example BFD its protected LAN adjacencies P, Q, and R, using, for example BFD
[RFC5880]. [RFC5880].
When S discovers that it has lost connectivity to P, it is unsure When S discovers that it has lost connectivity to P, it is unsure
whether the failure is: whether the failure is:
o its own interface to the LAN, o its own interface to the LAN,
o the LAN itself, o the LAN itself,
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Whilst this approach is simple, it assumes that a large portion of Whilst this approach is simple, it assumes that a large portion of
the network adjacent to the failure has also failed. This will the network adjacent to the failure has also failed. This will
result in the use of sub-optimal repair paths and in some cases the result in the use of sub-optimal repair paths and in some cases the
inability to identify a viable repair. inability to identify a viable repair.
5.2.2. LAN Component Repair 5.2.2. LAN Component Repair
In this approach, possible failures are considered at a finer In this approach, possible failures are considered at a finer
granularity, but without the use of diagnostics to identify the granularity, but without the use of diagnostics to identify the
specific component that has failed. Because S is unable to diagnose specific component that has failed. Because S is unable to diagnose
the failure it must repair traffic sent through P and B, to B not- the failure it MUST repair traffic sent through P and B, to B not-
via P,N (i.e. not via P and not via N), on the conservative via P,N (i.e. not via P and not via N), on the conservative
assumption that both the entire LAN and P have failed. Destinations assumption that both the entire LAN and P have failed. Destinations
for which P is a single point of failure must as usual be sent to P for which P is a single point of failure MUST as usual be sent to P
using an address that avoids the interface by which P is reached from using an address that avoids the interface by which P is reached from
S, i.e. to P not-via N. Similarly for routers Q and R. S, i.e. to P not-via N. Similarly for routers Q and R.
Notice that each router that is connected to a LAN must, as usual, Notice that each router that is connected to a LAN MUST, as usual,
advertise one not-via address for each neighbor. In addition, each advertise one not-via address for each neighbor. In addition, each
router on the LAN must advertise an extra address not via the router on the LAN MUST advertise an extra address not via the
pseudonode (N). pseudonode (N).
Notice also that each neighbor of a router connected to a LAN must Notice also that each neighbor of a router connected to a LAN MUST
advertise two not-via addresses, the usual one not via the neighbor advertise two not-via addresses, the usual one not via the neighbor
and an additional one, not via either the neighbor or the pseudonode. and an additional one, not via either the neighbor or the pseudonode.
The required set of LAN address assignments is shown in Figure 12 The required set of LAN address assignments is shown in Figure 12
below. Each router on the LAN, and each of its neighbors, is below. Each router on the LAN, and each of its neighbors, is
advertising exactly one address more than it would otherwise have advertising exactly one address more than it would otherwise have
advertised if this degree of connectivity had been achieved using advertised if this degree of connectivity had been achieved using
point-to-point links. point-to-point links.
Qs Qp Qc Cqn Qs Qp Qc Cqn
+--------------Q---------C +--------------Q---------C
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reverts to normal convergence. Although safe, this approach is reverts to normal convergence. Although safe, this approach is
somewhat draconian, since there are many circumstances were multiple somewhat draconian, since there are many circumstances were multiple
repairs do not induce loops. repairs do not induce loops.
This section describes the properties of multiple unrelated failures This section describes the properties of multiple unrelated failures
and proposes some methods that may be used to address this problem. and proposes some methods that may be used to address this problem.
5.3.1. Looping Repairs 5.3.1. Looping Repairs
Let us assume that the repair mechanism is based on solely on not-via Let us assume that the repair mechanism is based on solely on not-via
repairs. LFA or downstream routes may be incorporated, and will be repairs. LFA or downstream routes MAY be incorporated, and will be
dealt with later. dealt with later.
A------//------B------------D A------//------B------------D
/ \ / \
/ \ / \
F G F G
\ / \ /
\ / \ /
X------//------Y X------//------Y
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The ordering of the pairs in the lists is important. i.e. X-Y and The ordering of the pairs in the lists is important. i.e. X-Y and
Y-X are dealt with separately. If and only if the repairs are Y-X are dealt with separately. If and only if the repairs are
mutually incompatible, we need to advertise the pair of links as a mutually incompatible, we need to advertise the pair of links as a
secondary SRLG, and then ALL nodes compute repair paths around both secondary SRLG, and then ALL nodes compute repair paths around both
failures using an additional not-via address with the semantics not- failures using an additional not-via address with the semantics not-
via A-B AND not-via X-Y. via A-B AND not-via X-Y.
A further possibility is that because we are going to the trouble of A further possibility is that because we are going to the trouble of
advertising these SRLG sets, we could also advertise the new repair advertising these SRLG sets, we could also advertise the new repair
path and only get the nodes on that path to perform the necessary path and only get the nodes on that path to perform the necessary
computation. Note also that once we have reached Q space with computation. Note also that once we have reached Q-space Appendix A
respect to the two failures we need no longer continue the with respect to the two failures we need no longer continue the
computation, so we only need to notify the nodes on the path that are computation, so we only need to notify the nodes on the path that are
not in Q-space. not in Q-space.
One cause of mutually looping repair paths is the existence of nodes One cause of mutually looping repair paths is the existence of nodes
with only two links, or sections of the network which are only bi- with only two links, or sections of the network which are only bi-
connected. In these cases, repair is clearly impossible - the connected. In these cases, repair is clearly impossible - the
failure of both links partitions the network. It would be failure of both links partitions the network. It would be
advantageous to be able to identify these cases, and inhibit the advantageous to be able to identify these cases, and inhibit the
fruitless advertisement of the secondary SRLG information. This fruitless advertisement of the secondary SRLG information. This
could be achieved by the node detecting the requirement for a could be achieved by the node detecting the requirement for a
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downstream routes, and thus when the mutual pair consists of a downstream routes, and thus when the mutual pair consists of a
downstream route and a not-via repair, the looping packet will only downstream route and a not-via repair, the looping packet will only
be dropped when it gets back to the first failure. i.e. it will be dropped when it gets back to the first failure. i.e. it will
execute a single turn of the loop before being dropped. execute a single turn of the loop before being dropped.
There is a further complication with downstream routes, since There is a further complication with downstream routes, since
although the path may be computed to the far side of the failure, the although the path may be computed to the far side of the failure, the
packet may "peel off" to its destination before reaching the far side packet may "peel off" to its destination before reaching the far side
of the failure. In this case it may traverse some other link which of the failure. In this case it may traverse some other link which
has failed and was not accounted for on the computed path. If the has failed and was not accounted for on the computed path. If the
A-B repair (Figure 1) is a downstream route and the X-Y repair is a A-B repair (Figure 13) is a downstream route and the X-Y repair is a
not-via repair, we can have the situation where the X-Y repair not-via repair, we can have the situation where the X-Y repair
packets encapsulated to Yx follow a path which attempts to traverse packets encapsulated to Yx follow a path which attempts to traverse
A-B. If the A-B repair path for "normal" addresses is a downstream A-B. If the A-B repair path for "normal" addresses is a downstream
route, it cannot be assumed that the repair path for packets route, it cannot be assumed that the repair path for packets
addressed to Yx can be sent to the same neighbour. This is because addressed to Yx can be sent to the same neighbour. This is because
the validity of a downstream route must be ascertained in the the validity of a downstream route MUST be ascertained in the
topology represented by Yx, i.e. that with the link X-Y failed. This topology represented by Yx, i.e. that with the link X-Y failed. This
is not the same topology that was used for the normal downstream is not the same topology that was used for the normal downstream
calculation, and use of the normal downstream route for the calculation, and use of the normal downstream route for the
encapsulated packets may result in an undetected loop. If it is encapsulated packets may result in an undetected loop. If it is
computationally feasible to check the downstream route in this computationally feasible to check the downstream route in this
topology (i.e. for any not-via address Qp which traverses A-B we must topology (i.e. for any not-via address Qp which traverses A-B we MUST
perform the downstream calculation for that not-via address in the perform the downstream calculation for that not-via address in the
topology with link Q-P failed.), then the downstream repair for Yx topology with link Q-P failed.), then the downstream repair for Yx
can safely be used. These packets cannot re-visit X-Y, since by can safely be used. These packets cannot re-visit X-Y, since by
definition they will avoid that link. Alternatively, the packet definition they will avoid that link. Alternatively, the packet
could be always repaired in a not-via tunnel. i.e. even though the could be always repaired in a not-via tunnel. i.e. even though the
normal repair for traffic traversing A-B would be to use a downstream normal repair for traffic traversing A-B would be to use a downstream
route, we could insist that such traffic addressed to a not-via route, we could insist that such traffic addressed to a not-via
address must use a tunnel to Ba. Such a tunnel would only be address MUST use a tunnel to Ba. Such a tunnel would only be
installed for an address Qp if it were established that it did not installed for an address Qp if it were established that it did not
traverse Q-P (using the rules described above). traverse Q-P (using the rules described above).
6. Optimizing not-via computations using LFAs 6. Optimizing not-via computations using LFAs
If repairing node S has an LFA to the repair endpoint it is not If repairing node S has an LFA to the repair endpoint it is not
necessary for any router to perform the incremental SPF with the link necessary for any router to perform the incremental SPF with the link
SP removed in order to compute the route to the not-via address Ps. SP removed in order to compute the route to the not-via address Ps.
This is because the correct routes will already have been computed as This is because the correct routes will already have been computed as
a result of the SPF on the base topology. Node S can signal this a result of the SPF on the base topology. Node S can signal this
skipping to change at page 25, line 39 skipping to change at page 26, line 39
packet, and then pushing the label corresponding to the not-via packet, and then pushing the label corresponding to the not-via
address needed to effect the repair. Referring once again to address needed to effect the repair. Referring once again to
Figure 1, if S has a packet destined for D that it must reach via P Figure 1, if S has a packet destined for D that it must reach via P
and B, S first pushes B's label for D. S then pushes the label that and B, S first pushes B's label for D. S then pushes the label that
its next hop to Bp needs to reach Bp. its next hop to Bp needs to reach Bp.
Note that in an MPLS LDP network it is necessary for S to have the Note that in an MPLS LDP network it is necessary for S to have the
repair endpoint's label for the destination. When S is effecting a repair endpoint's label for the destination. When S is effecting a
link repair it already has this. In the case of a node repair, S link repair it already has this. In the case of a node repair, S
either needs to set up a directed LDP session with each of its either needs to set up a directed LDP session with each of its
neighbor's neighbors, or it needs to use the next-next hop label neighbor's neighbors, or it needs to use a method similar to the
distribution mechanism proposed in [I-D.shen-mpls-ldp-nnhop-label]. next-next hop label distribution mechanism proposed in
[I-D.shen-mpls-ldp-nnhop-label].
9. Encapsulation 9. Encapsulation
Any IETF specified IP in IP encapsulation may be used to carry a not- Any IETF specified IP in IP encapsulation may be used to carry a not-
via repair. IP in IP [RFC2003], GRE [RFC1701] and L2TPv3 [RFC3931], via repair. IP in IP [RFC2003], GRE [RFC1701] and L2TPv3 [RFC3931],
all have the necessary and sufficient properties. The requirement is all have the necessary and sufficient properties. The requirement is
that both the encapsulating router and the router to which the that both the encapsulating router and the router to which the
encapsulated packet is addressed have a common ability to process the encapsulated packet is addressed have a common ability to process the
chosen encapsulation type. When an MPLS LDP network is being chosen encapsulation type. When an MPLS LDP network is being
protected, the encapsulation would normally be an additional MPLS protected, the encapsulation would normally be an additional MPLS
label. In an MPLS enabled IP network an MPLS label may be used in label. In an MPLS enabled IP network an MPLS label may be used in
place of an IP in IP encapsulation in the case above. place of an IP in IP encapsulation in the case above.
10. Routing Extensions 10. Routing Extensions
IPFRR requires IGP extensions. Each IPFRR router that is directly IPFRR requires routing protocol extensions. Each IPFRR router that
connected to a protected network component must advertise a not-via is directly connected to a protected network component MUST advertise
address for that component. This must be advertised in such a way a not-via address for that component. This MUST be advertised in
that the association between the protected component (link, router or such a way that the association between the protected component
SRLG) and the not-via address can be determined by the other routers (link, router or SRLG) and the not-via address can be determined by
in the network. the other routers in the network.
It is necessary that not-via capable routers advertise in the IGP It is necessary that not-via capable routers advertise in the IGP
that they will calculate not-via routes. that they will calculate not-via routes.
It is necessary for routers to advertise the type of encapsulation It is necessary for routers to advertise the type of encapsulation
that they support (MPLS, GRE, L2TPv3 etc). However, the deployment that they support (MPLS, GRE, L2TPv3 etc). However, the deployment
of mixed IP encapsulation types within a network is discouraged. of mixed IP encapsulation types within a network is discouraged.
If the optimization proposed in Section 6 is to be used the use of
the LFA in place of the not-via repair MUST also be signalled in the
routing protocol.
11. Incremental Deployment 11. Incremental Deployment
Incremental deployment is supported by excluding routers that are not Incremental deployment is supported by excluding routers that are not
calculating not-via routes (as indicated by their capability calculating not-via routes (as indicated by their capability
information flooded with their link state information) from the base information flooded with their link state information) from the base
topology used for the computation of repair paths. In that way topology used for the computation of repair paths. In that way
repairs may be steered around islands of routers that are not IPFRR repairs may be steered around islands of routers that are not IPFRR
capable. Routers that are protecting a network component need to capable. Routers that are protecting a network component need to
have the capability to encapsulate and decapsulate packets. However, have the capability to encapsulate and decapsulate packets. However,
routers that are on the repair path only need to be capable of routers that are on the repair path only need to be capable of
calculating not-via paths and including the not-via addresses in calculating not-via paths and including the not-via addresses in
their FIB i.e. these routers do not need any changes to their their FIB i.e. these routers do not need any changes to their
forwarding mechanism. forwarding mechanism.
12. IANA Considerations 12. Manageability Considerations
[RFC5714] outlines the general set of manageability consideration
that apply to the general case of IPFRR. We slightly expand this and
add details that are not-via specific. There are three classes
manageability consideration:
1. Pre-failure configuration
2. Pre-failure Monitoring and operational support
3. Failure action verification
12.1. Pre-failure configuration
Pre-failure configuration for not-via includes:
o Enabling/disabling not-via IPFRR support.
o Enabling/disabling protection on a per-link or per-node basis.
o Expressing preferences regarding the links/nodes used for repair
paths.
o Configuration of failure detection mechanisms.
o Setting a preference concerning the use of LFA.
o Configuring not-via address (per interface), or not-via address
set (per node).
o Configuring any SRLG rules or preferences.
Any standard configuration method may be used and the selection of
the method to be used is outside the scope of this document.
12.2. Pre-failure Monitoring and operational support
Pre-failure Monitoring and operational support for not-via includes:
o Notification of links/nodes/destinations that cannot be protected.
o Notification of pre-computed repair paths.
o Notification of repair type to be used (LFA or not-via).
o Notification of not-via address assignment.
o Notification of path or address optimizations used.
o Testing repair paths. Note that not-via addresses look identical
to "ordinary" addresses as far as tools such as trace route and
ping are concerned and thus it is anticipated that these will be
used to verify the established repair path.
Any standard IETF method may be used for the above and the selection
of the method to be used is outside the scope of this document.
12.3. Failure action monitoring
Failure action monitoring for not-via includes:
o Counts of failure detections, protection invocations, and packets
forwarded over repair paths.
o Logging of the events using a sufficiently accurate and precise
timestamp.
o Validation that the packet loss was within specification using a
suitable loss verification tool.
o Capture of the in-flight repair packet flows using a tool such as
IPFIX[RFC5101].
Note that monitoring the repair in action requires the capture of the
signatures of a short, possibly sub-second network transient which is
not a well developed IETF technology.
13. IANA Considerations
There are no IANA considerations that arise from this draft. There are no IANA considerations that arise from this draft.
13. Security Considerations 14. Security Considerations
The repair endpoints present vulnerability in that they might be used The repair endpoints present vulnerability in that they might be used
as a method of disguising the delivery of a packet to a point in the as a method of disguising the delivery of a packet to a point in the
network. The primary method of protection should be through the use network. The primary method of protection SHOULD be through the use
of a private address space for the not-via addresses. These of a private address space for the not-via addresses. These
addresses must not be advertised outside the area, and should be addresses MUST NOT be advertised outside the area, and SHOULD be
filtered at the network entry points. In addition, a mechanism might filtered at the network entry points. In addition, a mechanism might
be developed that allowed the use of the mild security available be developed that allowed the use of the mild security available
through the use of a key [RFC1701] [RFC3931]. With the deployment of through the use of a key [RFC1701] [RFC3931]. With the deployment of
such mechanisms, the repair endpoints would not increase the security such mechanisms, the repair endpoints would not increase the security
risk beyond that of existing IP tunnel mechanisms. An attacker may risk beyond that of existing IP tunnel mechanisms. An attacker may
attempt to overload a router by addressing an excessive traffic load attempt to overload a router by addressing an excessive traffic load
to the de-capsulation endpoint. Typically, routers take a 50% to the de-capsulation endpoint. Typically, routers take a 50%
performance penalty in decapsulating a packet. The attacker could performance penalty in decapsulating a packet. The attacker could
not be certain that the router would be impacted, and the extremely not be certain that the router would be impacted, and the extremely
high volume of traffic needed, would easily be detected as an high volume of traffic needed, would easily be detected as an
anomaly. If an attacker were able to influence the availability of a anomaly. If an attacker were able to influence the availability of a
link, they could cause the network to invoke the not-via repair link, they could cause the network to invoke the not-via repair
mechanism. A network protected by not-via IPFRR is less vulnerable mechanism. A network protected by not-via IPFRR is less vulnerable
to such an attack than a network that undertook a full convergence in to such an attack than a network that undertook a full convergence in
response to a link up/down event. response to a link up/down event.
14. Acknowledgements 15. Acknowledgements
The authors would like to acknowledge contributions made by Alia The authors would like to acknowledge contributions made by Alia
Atlas and John Harper. Atlas and John Harper.
15. Informative References 16. References
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
16.2. Informative References
[I-D.ietf-rtgwg-ordered-fib] [I-D.ietf-rtgwg-ordered-fib]
Shand, M., Bryant, S., Previdi, S., and C. Filsfils, Shand, M., Bryant, S., Previdi, S., and C. Filsfils,
"Loop-free convergence using oFIB", "Loop-free convergence using oFIB",
draft-ietf-rtgwg-ordered-fib-05 (work in progress), draft-ietf-rtgwg-ordered-fib-05 (work in progress),
April 2011. April 2011.
[I-D.shand-remote-lfa]
Bryant, S., Filsfils, C., Shand, M., and N. So, "Remote
LFA FRR", draft-shand-remote-lfa-01 (work in progress),
June 2012.
[I-D.shen-mpls-ldp-nnhop-label] [I-D.shen-mpls-ldp-nnhop-label]
Shen, N., "Discovering LDP Next-Nexthop Labels", Shen, N., "Discovering LDP Next-Nexthop Labels",
draft-shen-mpls-ldp-nnhop-label-02 (work in progress), draft-shen-mpls-ldp-nnhop-label-02 (work in progress),
May 2005. May 2005.
[ISPF] McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing [ISPF] McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing
Algorithm Improvements"", BBN Technical Report 3803, 1978. Algorithm Improvements"", BBN Technical Report 3803, 1978.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994. Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996. October 1996.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling [RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007. Specification", RFC 5036, October 2007.
[RFC5101] Claise, B., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008. Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010. RFC 5714, January 2010.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010. (BFD)", RFC 5880, June 2010.
Appendix A. Q-Space
Q-space is the set of routers from which a specific router can be
reached without any path (including equal cost path splits)
transiting the protected link (or node). It is fully described in
[I-D.shand-remote-lfa].
S---E
/ \
A D
\ /
B---C
Figure 15
Consider a repair of link S-E (Figure 15). The set of routers from
which the node E can be reached, by normal forwarding, without
traversing the link S-E is termed the Q-space of E with respect to
the link S-E. The Q-space can be obtained by computing a reverse
shortest path tree (rSPT) rooted at E, with the sub-tree which
traverses the failed link excised (including those which are members
of an ECMP). The rSPT uses the cost towards the root rather than
from it and yields the best paths towards the root from other nodes
in the network. In the case of Figure 15 the Q-space comprises nodes
C and D only.
Authors' Addresses Authors' Addresses
Stewart Bryant Stewart Bryant
Cisco Systems Cisco Systems
250, Longwater Avenue. 250, Longwater Avenue.
Reading, Berks RG2 6GB Reading, Berks RG2 6GB
UK UK
Email: stbryant@cisco.com Email: stbryant@cisco.com
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