draft-ietf-rtgwg-ipfrr-notvia-addresses-10.txt   draft-ietf-rtgwg-ipfrr-notvia-addresses-11.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 22, 2013 M. Shand Expires: November 25, 2013 M. Shand
Individual Contributor Individual Contributor
December 19, 2012 May 24, 2013
A Framework for IP and MPLS Fast Reroute Using Not-via Addresses A Framework for IP and MPLS Fast Reroute Using Not-via Addresses
draft-ietf-rtgwg-ipfrr-notvia-addresses-10 draft-ietf-rtgwg-ipfrr-notvia-addresses-11
Abstract Abstract
This document presents a framework for providing fast reroute in an This document presents an illustrative framework for providing fast
IP or MPLS network through encapsulation and forwarding to "not-via" reroute in an IP or MPLS network through encapsulation and forwarding
addresses. The general approach described uses a single level of to "not-via" addresses. The general approach described uses a single
encapsulation and could be used to protect unicast, multicast, and level of encapsulation and could be used to protect unicast,
LDP traffic against link, router, and shared risk group failure, multicast, and LDP traffic against link, router, and shared risk
regardless of network topology and metrics. group failure, regardless of network topology and metrics.
The mechanisms presented in this document are purely illustrative of The mechanisms presented in this document are purely illustrative of
the general approach and do not constitute a protocol specification. the general approach and do not constitute a protocol specification.
The document represents a snapshot of the work of the Routing Area The document represents a snapshot of the work of the Routing Area
Working Group at the time of publication and is published as a Working Group at the time of publication and is published as a
document of record. Further work is needed before implementation or document of record. Further work is needed before implementation or
deployment. deployment.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119]. 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 22, 2013. This Internet-Draft will expire on November 25, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. The Purpose of this Document . . . . . . . . . . . . . . . . . 4 1. The Purpose of this Document . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 5 3. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 4
3.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . . 6 3.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . 5
3.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . . 6 3.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . 5
4. Not-via Repair Path Computation . . . . . . . . . . . . . . . 7 4. Not-via Repair Path Computation . . . . . . . . . . . . . . . 6
4.1. Computing not-via repairs in distance and path vector 4.1. Computing not-via repairs in distance and path vector
routing protocols . . . . . . . . . . . . . . . . . . . . 8 routing protocols . . . . . . . . . . . . . . . . . . . . 7
5. Operation of Repairs . . . . . . . . . . . . . . . . . . . . . 8 5. Operation of Repairs . . . . . . . . . . . . . . . . . . . . 7
5.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . . 9 5.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . 8
5.2.1. Loop Prevention Under Node Failure . . . . . . . . . . 9 5.2.1. Loop Prevention Under Node Failure . . . . . . . . . 8
5.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9 5.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . 8
5.4. Installation of Repair Paths . . . . . . . . . . . . . . . 11 5.4. Installation of Repair Paths . . . . . . . . . . . . . . 10
6. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 12 6. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 12 6.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 11
6.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 16 6.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 16
6.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 17 6.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 16
6.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 18 6.2.2. LAN Component Repair . . . . . . . . . . . . . . . . 17
6.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . . 19 6.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . 18
6.3. Multiple Independent Failures . . . . . . . . . . . . . . 19 6.3. Multiple Independent Failures . . . . . . . . . . . . . . 18
6.3.1. Looping Repairs . . . . . . . . . . . . . . . . . . . 20 6.3.1. Looping Repairs . . . . . . . . . . . . . . . . . . . 19
6.3.2. Outline Solution . . . . . . . . . . . . . . . . . . . 21 6.3.2. Outline Solution . . . . . . . . . . . . . . . . . . 20
6.3.3. Looping Repairs . . . . . . . . . . . . . . . . . . . 22 6.3.3. Looping Repairs . . . . . . . . . . . . . . . . . . . 21
6.3.3.1. Dropping Looping Packets . . . . . . . . . . . . . 22 6.3.3.1. Dropping Looping Packets . . . . . . . . . . . . 21
6.3.3.2. Computing non-looping Repairs of Repairs . . . . . 23 6.3.3.2. Computing non-looping Repairs of Repairs . . . . 22
6.3.4. Mixing LFAs and Not-via . . . . . . . . . . . . . . . 25 6.3.4. Mixing LFAs and Not-via . . . . . . . . . . . . . . . 24
7. Optimizing not-via computations using LFAs . . . . . . . . . . 26 7. Optimizing not-via computations using LFAs . . . . . . . . . 25
8. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 27 8. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 27 9. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . 26
10. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 27 10. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 26
11. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 28 11. Routing Extensions . . . . . . . . . . . . . . . . . . . . . 27
12. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 28 12. Incremental Deployment . . . . . . . . . . . . . . . . . . . 27
13. Manageability Considerations . . . . . . . . . . . . . . . . . 28 13. Manageability Considerations . . . . . . . . . . . . . . . . 27
13.1. Pre-failure configuration . . . . . . . . . . . . . . . . 29 13.1. Pre-failure configuration . . . . . . . . . . . . . . . 28
13.2. Pre-failure Monitoring and operational support . . . . . . 29 13.2. Pre-failure Monitoring and operational support . . . . . 28
13.3. Failure action monitoring . . . . . . . . . . . . . . . . 30 13.3. Failure action monitoring . . . . . . . . . . . . . . . 29
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
15. Security Considerations . . . . . . . . . . . . . . . . . . . 30 15. Security Considerations . . . . . . . . . . . . . . . . . . . 29
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31 16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
17.1. Normative References . . . . . . . . . . . . . . . . . . . 31 17.1. Normative References . . . . . . . . . . . . . . . . . . 30
17.2. Informative References . . . . . . . . . . . . . . . . . . 31 17.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Q-Space . . . . . . . . . . . . . . . . . . . . . . . 32 Appendix A. Q-Space . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. The Purpose of this Document 1. The Purpose of this Document
This document presents a framework for providing fast re-route around This document presents an illustrative framework for providing fast
a failure in an IP or MPLS network based on the concept tunnelling or re-route around a failure in an IP or MPLS network based on the
encapsulating packets via an IP address that is known to avoid the concept of tunnelling or encapsulating packets via an IP address that
failure. The general approach described uses a single level of is known to avoid the failure. The general approach described uses a
encapsulation and could be used to protect unicast, multicast, and single level of encapsulation and could be used to protect unicast,
LDP traffic against link, router, and shared risk group failure, multicast, and LDP traffic against link, router, and shared risk
regardless of network topology and metrics. group failure, regardless of network topology and metrics.
At the time of publication there is no demand to deploy this At the time of publication there is no demand to deploy this
technology, however in view of the subtleties involved in the design technology, however in view of the subtleties involved in the design
of routing protocol extensions to provide IP Fast Reroute (IPFRR) the of routing protocol extensions to provide IP Fast Reroute (IPFRR) the
Routing Area Working Group considered it desirable to publish this Routing Area Working Group considered it desirable to publish this
document to place on record the design consideration of the not-via document to place on record the design consideration of the not-via
address approach. address approach.
The mechanisms presented in this document are purely illustrative of The mechanisms presented in this document are purely illustrative of
the general approach and do not constitute a protocol specification. the general approach and do not constitute a protocol specification.
skipping to change at page 5, line 12 skipping to change at page 4, line 24
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.
3. Overview of Not-via Repairs 3. 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. Consider the network fragment shown in Figure 1 below, in IPFRR. Consider the network fragment shown in Figure 1 below, in
which S has a packet for some destination D that it would normally 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. 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 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
In the not-via IPFRR method, S encapsulates the packet to Bp, where In the not-via IPFRR method, S encapsulates the packet to Bp, where
Bp is an address on node B that has the property that it is not Bp is an address on node B that has the property that it is not
reachable from node P, i.e. the notation Bp means "an address of node reachable from node P, i.e. the notation Bp means "an address of
B that is only reachable not via node P". We later show how to node B that is only reachable not via node P". We later show how to
install the path from S to Bp such that it is the shortest path from install the path from S to Bp such that it is the shortest path from
S to B not going via P. If the network contains a path from S to B S to B not going via P. If the network contains a path from S to B
that does not transit router P, i.e. the network is not partitioned that does not transit router P, i.e. the network is not partitioned
by the failure of P and the path from S to Bp has been installed, by the failure of P and the path from S to Bp has been installed,
then the packet will be successfully delivered to B. In the example then the 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 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 addressed to Bp arrives at B, B removes the encapsulation and
forwards the repaired packet towards its final destination. 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. Similarly, P will require a set of 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 not-via address (one for each neighbor) allowing traffic to be
directed to P without traversing each of those neighbors. directed to P without traversing each of those neighbors.
The not-via addresses are advertised in the routing protocol in a way The not-via addresses are advertised in the routing protocol in a way
that clearly identifies them as not-via addresses and not 'ordinary' that clearly identifies them as not-via addresses and not 'ordinary'
addresses. 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
3.1. Use of Equal Cost Multi-Path 3.1. 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.
3.2. Use of LFA repairs 3.2. 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]. In (LFA) and or downstream paths as documented in [RFC5286]. In
particular LFAs do not require the assignment and management of particular LFAs do not require the assignment and management of
additional IP addresses to nodes, they do not require nodes in the additional IP addresses to nodes, they do not require nodes in the
network to be upgraded in order to calculate not-via repair paths, network to be upgraded in order to calculate not-via repair paths,
and they do not require the use of encapsulation. 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
skipping to change at page 8, line 15 skipping to change at page 7, line 27
section 6. section 6.
4.1. Computing not-via repairs in distance and path vector routing 4.1. Computing not-via repairs in distance and path vector routing
protocols 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 needs to be suppressed not
across the link N->P, but also across any link to P. The simplest way only across the link N->P, but also across any link to P. The
of achieving this is for P itself to perform the suppression of any simplest way of achieving this is for P itself to perform the
address of the form Xp. suppression of any address of the form Xp.
5. Operation of Repairs 5. 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.
5.1. Node Failure 5.1. 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
skipping to change at page 9, line 13 skipping to change at page 8, line 23
clearly impossible. clearly impossible.
5.2. Link Failure 5.2. 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
network to deliver the packet to P not-via S). All of the neighbors the network to deliver the packet to P not-via S). All of the
of S will have calculated a path to Ps in case S itself had failed. neighbors of S will have calculated a path to Ps in case S itself had
S could therefore give the packet to any of its neighbors (except, of failed. S could therefore give the packet to any of its neighbors
course, P). However, S SHOULD send the encapsulated packet on the (except, of course, P). However, S SHOULD send the encapsulated
shortest available path to P. This path is calculated by running an packet on the shortest available path to P. This path is calculated
SPF with the link SP failed. Note that this may again be an by running an SPF with the link SP failed. Note that this may again
incremental calculation, which can terminate when address Ps has been be an incremental calculation, which can terminate when address Ps
reattached. has been reattached.
5.2.1. Loop Prevention Under Node Failure 5.2.1. 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 as a result of mutual repair, by never providing a repair path loops as a result of mutual repair, by never providing a repair path
for a not-via address. The repair of packets with not-via addresses for a not-via address. The repair of packets with not-via addresses
is considered in more detail in Section 6.3. Referring to Figure 2, is considered in more detail in Section 6.3. Referring to Figure 2,
if A was the neighbor of P that was on the link repair path from S to if A was the neighbor of P that was on the link repair path from S to
skipping to change at page 10, line 5 skipping to change at page 9, line 11
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
through P, to an alternate router, which is still able to reach X. through P, to an alternate router, which is still able to reach X.
X X X X X X
| | | | | |
| | | | | |
| 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
normal address for Z). When Z receives the encapsulated packet it normal address for Z). When Z receives the encapsulated packet it
removes the encapsulation and forwards the packet to X. removes the encapsulation and forwards the packet to X.
Now consider the case where the shortest alternate path to X is via Now consider the case where the shortest alternate path to X is via
Y, which S reaches via P and B. To reach Y, S must first repair the Y, which S reaches via P and B. To reach Y, S must first repair the
packet to B using the normal not-via repair mechanism. To do this S packet to B using the normal not-via repair mechanism. To do this S
encapsulates the packet for X to Bp. When B receives the packet it encapsulates the packet for X to Bp. When B receives the packet it
removes the encapsulation and discovers that the packet is intended removes the encapsulation and discovers that the packet is intended
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.
5.4. Installation of Repair Paths 5.4. 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
calculate and install repair paths in the Forwarding Information Base and install repair paths in the Forwarding Information Base (FIB),
(FIB), ready for immediate use in the event of a failure. It is ready for immediate use in the event of a failure. It is assumed
assumed that the not-via repair paths have already been calculated as that the not-via repair paths have already been calculated as
described above. described above.
For each neighbor P, consider all destinations which are reachable For each neighbor P, consider all destinations which are reachable
via P in the current topology:- via P in the current topology:-
1. For all destinations with an ECMP or LFA repair (as described in 1. For all destinations with an ECMP or LFA repair (as described in
[RFC5286]) install that repair. [RFC5286]) install that repair.
2. For each destination (DR) that remains, identify in the current 2. For each destination (DR) that remains, identify in the current
topology the next-next-hop (H) (i.e. the neighbor of P that P topology the next-next-hop (H) (i.e. the neighbor of P that P
will use to send the packet to DR). This can be determined will use to send the packet to DR). This can be determined
during the normal SPF run by recording the additional during the normal SPF run by recording the additional
information. If S has a path to the not-via address Hp (H not information. If S has a path to the not-via address Hp (H not
via P), install a not-via repair to Hp for the destination DR. via P), install a not-via repair to Hp for the destination DR.
3. Identify all remaining destinations (M) which can still be 3. Identify all remaining destinations (M) which can still be
reached when node P fails. These will be multi-homed prefixes reached when node P fails. These will be multi-homed prefixes
that are not repairable by LFA, for which the normal attachment that are not repairable by LFA, for which the normal attachment
node is P, or a router for which P is a single point of failure, node is P, or a router for which P is a single point of failure,
and have an alternative attachment point that is reachable after and have an alternative attachment point that is reachable after
P has failed. One way of determining these destinations would be P has failed. One way of determining these destinations would be
to run an SPF rooted at S with node P removed, but an to run an SPF rooted at S with node P removed, but an
implementation may record alternative attachment points during implementation may record alternative attachment points during
the normal SPF run. In either case, the next best point of the normal SPF run. In either case, the next best point of
attachment can also be determined for use in step (4) below. attachment can also be determined for use in step (4) below.
4. For each multi-homed prefix (M) identified in step (3):- 4. For each multi-homed prefix (M) identified in step (3):-
A. Identify the new attachment node (as shown in Figure 3). a. Identify the new attachment node (as shown in Figure 3).
This may be:- This may be:-
a. Y, where the next hop towards Y is P, or a. Y, where the next hop towards Y is P, or
b. Z, where the next hop towards Z is not P. b. Z, where the next hop towards Z is not P.
If the attachment node is Z, install the repair for M as a If the attachment node is Z, install the repair for M as a
tunnel to Z' (where Z' is the address of Z that is used to tunnel to Z' (where Z' is the address of Z that is used to
force local forwarding). force local forwarding).
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.
Note that where multiple next-next-hops are available to reach DR,
any or several of them may be chosen from a routing correctness point
of view. Unless other factors require consideration the closest
next-next-hop to the repairing router would be the normal choice.
6. Compound Failures 6. Compound Failures
The following types of failures involve more than one component: The following types of failures involve more than one component:
1. Shared Risk Link Groups 1. Shared Risk Link Groups
2. Local Area Networks 2. Local Area Networks
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.
6.1. Shared Risk Link Groups 6.1. 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 needs to be computed to avoid not only the adjacent link, but also
the links which are members of the same SRLG. all 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
links in the SRLG when running the iSPF. links in the SRLG when running the iSPF.
Note that it is not necessary for S to consider repairs to any other Note that it is not necessary for S to consider repairs to any other
nodes attached to members of the SRLG (such as B). It is sufficient nodes attached to members of the SRLG (such as B). It is sufficient
for S to repair to the other end of the adjacent link (P in this for S to repair to the other end of the adjacent link (P in this
case). case).
a Ps a Ps
S----------P---------D S----------P---------D
| | | |
| a | | a |
A----------B A----------B
| | | |
| | | |
C----------E C----------E
Figure 4: Shared Risk Link Group Figure 4: Shared Risk Link Group
In some cases, it may be that the links comprising the SRLG occur in In some cases, it may be that the links comprising the SRLG occur in
series on the path from S to the destination D, as shown in Figure 5. series on the path from S to the destination D, as shown in Figure 5.
In this case, multiple consecutive repairs may be necessary. S will In this case, multiple consecutive repairs may be necessary. S will
first repair to Ps, then P will repair to Dp. In both cases, because first repair to Ps, then P will repair to Dp. In both cases, because
the links concerned are members of SRLG "a" the paths are computed to the links concerned are members of SRLG "a" the paths are computed to
avoid all members of SRLG "a". avoid all members of SRLG "a".
a Ps a Dp a Ps a Dp
S----------P---------D S----------P---------D
| | | | | |
| a | | | a | |
A----------B | A----------B |
| | | | | |
| | | | | |
C----------E---------F C----------E---------F
Figure 5: Shared Risk Link Group members in series Figure 5: Shared Risk Link Group members in series
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 decapsulation 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 | |
| | | | | | | |
| | | | | | | |
C----------E---------F--------H C----------E---------F--------H
Figure 6: Shared Risk Link Group members in series Figure 6: Shared Risk Link Group members in series
In this case, S first encapsulates to Ps, and node P decapsulates the In this case, S first encapsulates to Ps, and node P decapsulates the
packet and forwards it "native" to G using its normal FIB entry for packet and forwards it "native" to G using its normal FIB entry for
destination D. G then repairs the packet to Dg. destination D. G then repairs the packet to Dg.
It can be shown that such multiple repairs can never form a loop It can be shown that such multiple repairs can never form a loop
because each repair causes the packet to move closer to its because each repair causes the packet to move closer to its
destination. destination.
It is often the case that a single link may be a member of multiple It is often the case that a single link may be a member of multiple
SRLGs, and those SRLGs may not be isomorphic. This is illustrated in SRLGs, and those SRLGs may not be isomorphic. This is illustrated in
Figure 7 below. Figure 7 below.
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 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
links which are members of SRLG "a" or SRLG "b". I.e. the semantics Therefore the not-via path to Ps needs to be computed by failing all
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 | | |
A----/-----B | | A----/-----B | |
| | | | | | | |
| b | | b | | b | | b |
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, which would be an instance of multiple "b" have in fact failed, which would be an instance of multiple
independent failures. In practice, it is likely that there is only a independent failures. In practice, it is likely that there is only a
single failure, i.e. either SRLG "a" or SRLG "b" has failed, but not single failure, i.e. either SRLG "a" or SRLG "b" has failed, but not
both. These two possibilities are indistinguishable from the point both. These two possibilities are indistinguishable from the point
of view of the repairing router S and so it MUST repair on the of view of the repairing router S and so it needs to repair on the
assumption that both are unavailable. However, each link repair is assumption that both are unavailable. However, each link repair is
considered independently. The repair to Ps delivers the packet to P considered independently. The repair to Ps delivers the packet to P
which then forwards the packet to G. When the packet arrives at G, if which then forwards the packet to G. When the packet arrives at G,
SRLG "a" has failed it will be repaired around the path G-F-H-D. if SRLG "a" 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 This is illustrated in Figure 9 below. If, on the other hand, SRLG
"b" has failed, link GD will still be available. In this case the "b" has failed, link GD will still be available. In this case the
packet 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 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 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 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 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, 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 because packets addressed to a not-via address are not repaired in
basic not-via IPFRR. basic not-via IPFRR.
The repair of multiple independent failures is not provided by the The repair of multiple independent failures is not provided by the
basic not-via IPFRR method described so far in this memo. 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
skipping to change at page 17, line 5 skipping to change at page 16, line 18
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.
+--------------Q------C +--------------Q------C
| |
| |
| |
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 needs to run a connectivity check to each
its protected LAN adjacencies P, Q, and R, using, for example BFD of 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,
o the LAN interface of P, o the LAN interface of P,
o the node P. o the node P.
6.2.1. Simple LAN Repair 6.2.1. Simple LAN Repair
A simple approach to LAN repair is to consider the LAN and all of its A simple approach to LAN repair is to consider the LAN and all of its
connected routers as a single SRLG. Thus, the address P not via the connected routers as a single SRLG. Thus, the address P not via the
LAN (Pl) would require P to be reached not-via any router connected LAN (Pl) would require P to be reached not-via any router connected
to the LAN. This is shown in Figure 11. to the LAN. This is shown in Figure 11.
Ql Cl Ql Cl
+-------------Q--------C +-------------Q--------C
| Qc | Qc
| |
As Sl | Pl Bl As Sl | Pl Bl
A--------S-------(N)------------P--------B A--------S-------(N)------------P--------B
Sa | Pb Sa | Pb
| |
| Rl Dl | Rl Dl
+-------------R--------D +-------------R--------D
Rd Rd
Figure 11: Local Area Networks - LAN SRLG Figure 11: Local Area Networks - LAN SRLG
In this case, when S detected that P had failed it would send traffic In this case, when S detected that P had failed it would send traffic
reached via P and B to B not-via the LAN or any router attached to reached via P and B to B not-via the LAN or any router attached to
the LAN (i.e. to Bl). Any destination only reachable through P would the LAN (i.e. to Bl). Any destination only reachable through P
be addressed to P not-via the LAN or any router attached to the LAN would be addressed to P not-via the LAN or any router attached to the
(except of course P). LAN (except of course P).
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.
6.2.2. LAN Component Repair 6.2.2. 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 needs to repair traffic sent through P and B, to B
via P,N (i.e. not via P and not via N), on the conservative not- 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 needs
advertise two not-via addresses, the usual one not via the neighbor to advertise two not-via addresses, the usual one not via the
and an additional one, not via either the neighbor or the pseudonode. neighbor and an additional one, not via either the neighbor or the
The required set of LAN address assignments is shown in Figure 12 pseudonode. The required set of LAN address assignments is shown in
below. Each router on the LAN, and each of its neighbors, is Figure 12 below. Each router on the LAN, and each of its neighbors,
advertising exactly one address more than it would otherwise have is 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
| Qr Qn Cq | Qr Qn Cq
| |
Asn Sa Sp Sq | Ps Pq Pb Bpn Asn Sa Sp Sq | Ps Pq Pb Bpn
A--------S-------(N)-------------P---------B A--------S-------(N)-------------P---------B
As Sr Sn | Pr Pn Bp As Sr Sn | Pr Pn Bp
| |
| Rs Rp Pd Drn | Rs Rp Pd Drn
+--------------R---------D +--------------R---------D
Rq Rn Dr Rq Rn Dr
Figure 12: Local Area Networks Figure 12: Local Area Networks
6.2.3. LAN Repair Using Diagnostics 6.2.3. LAN Repair Using Diagnostics
A more specific LAN repair can be undertaken by using diagnostics. A more specific LAN repair can be undertaken by using diagnostics.
In order to explicitly diagnose the failed network component, S In order to explicitly diagnose the failed network component, S
correlates the connectivity reports from P and one or more of the correlates the connectivity reports from P and one or more of the
other routers on the LAN, in this case, Q and R. If it lost other routers on the LAN, in this case, Q and R. If it lost
connectivity to P alone, it could deduce that the LAN was still connectivity to P alone, it could deduce that the LAN was still
functioning and that the fault lay with either P, or the interface functioning and that the fault lay with either P, or the interface
connecting P to the LAN. It would then repair to B not via P (and P connecting P to the LAN. It would then repair to B not via P (and P
not-via N for destinations for which P is a single point of failure) not-via N for destinations for which P is a single point of failure)
in the usual way. If S lost connectivity to more than one router on in the usual way. If S lost connectivity to more than one router on
the LAN, it could conclude that the fault lay only with the LAN, and the LAN, it could conclude that the fault lay only with the LAN, and
could repair to P, Q and R not-via N, again in the usual way. could repair to P, Q and R not-via N, again in the usual way.
6.3. Multiple Independent Failures 6.3. Multiple Independent Failures
skipping to change at page 20, line 26 skipping to change at page 19, line 28
\ / \ /
\ / \ /
X------//------Y X------//------Y
Figure 13: The General Case of Multiple Failures Figure 13: The General Case of Multiple Failures
The essential case is as illustrated in Figure 13. Note that The essential case is as illustrated in Figure 13. Note that
depending on the repair case under consideration, there may be paths depending on the repair case under consideration, there may be paths
present in Figure 13, that are in addition to those shown in the present in Figure 13, that are in addition to those shown in the
figure. For example there may be paths between A and B, and/or figure. For example there may be paths between A and B, and/or
between X and Y. These paths are omitted for graphical clarity. between X and Y. These paths are omitted for graphical clarity.
There are three cases to consider: There are three cases to consider:
1) Consider the general case of a pair of protected links A-B and 1) Consider the general case of a pair of protected links A-B and
X-Y as shown in the network fragment shown Figure 13. If the X-Y as shown in the network fragment shown Figure 13. If the
repair path for A-B does not traverse X-Y and the repair path for repair path for A-B does not traverse X-Y and the repair path for
X-Y does not traverse A-B, this case is completely safe and will X-Y does not traverse A-B, this case is completely safe and will
not cause looping or packet loss. not cause looping or packet loss.
A more common variation of this case is shown in Figure 14, which A more common variation of this case is shown in Figure 14, which
shows two failures in different parts of the network in which a shows two failures in different parts of the network in which a
packet from A to D traverses two concatenated repairs. packet from A to D traverses two concatenated repairs.
A------//------B------------X------//------Y------D A------//------B------------X------//------Y------D
| | | | | | | |
| | | | | | | |
M--------------+ N--------------+ M--------------+ N--------------+
Figure 14: Concatenated Repairs Figure 14: Concatenated Repairs
2) In Figure 13, the repair for A-B traverses X-Y, but the repair 2) In Figure 13, the repair for A-B traverses X-Y, but the repair
for X-Y does not traverse A-B. This case occurs when the not-via for X-Y does not traverse A-B. This case occurs when the not-via
path from A to B traverses link X-Y, but the not-via path from X path from A to B traverses link X-Y, but the not-via path from X
to Y traverses some path not shown in Figure 13. Without the to Y traverses some path not shown in Figure 13. Without the
multi-failure mechanism described in this section the repaired multi-failure mechanism described in this section the repaired
packet for A-B would be dropped when it reached X-Y, since the packet for A-B would be dropped when it reached X-Y, since the
repair of repaired packets would be forbidden. However, if this repair of repaired packets would be forbidden. However, if this
packet were allowed to be repaired, the path to D would be packet were allowed to be repaired, the path to D would be
complete and no harm would be done, although two levels of complete and no harm would be done, although two levels of
skipping to change at page 21, line 33 skipping to change at page 20, line 33
3) The repair for A-B traverses X-Y AND the repair for X-Y 3) The repair for A-B traverses X-Y AND the repair for X-Y
traverses A-B. In this case unrestricted repair would result in traverses A-B. In this case unrestricted repair would result in
looping packets and increasing levels of encapsulation. looping packets and increasing levels of encapsulation.
The challenge in applying IPFRR to a network that is undergoing The challenge in applying IPFRR to a network that is undergoing
multiple failures is, therefore, to identify which of these cases multiple failures is, therefore, to identify which of these cases
exist in the network and react accordingly. exist in the network and react accordingly.
6.3.2. Outline Solution 6.3.2. Outline Solution
When A is computing the not-via repair path for A-B (i.e. the path When A is computing the not-via repair path for A-B (i.e. the path
for packets addressed to Ba, read as "B not-via A") it is aware of for packets addressed to Ba, read as "B not-via A") it is aware of
the list of nodes which this path traverses. This can be recorded by the list of nodes which this path traverses. This can be recorded by
a simple addition to the SPF process, and the not-via addresses a simple addition to the SPF process, and the not-via addresses
associated with each forward link can be determined. If the path associated with each forward link can be determined. If the path
were A, F, X, Y, G, B, (Figure 13) the list of not-via addresses were A, F, X, Y, G, B, (Figure 13) the list of not-via addresses
would be: Fa, Xf, Yx, Gy, Bg. Under standard not-via operation, A would be: Fa, Xf, Yx, Gy, Bg. Under standard not-via operation, A
would populate its FIB such that all normal addresses normally would populate its FIB such that all normal addresses normally
reachable via A-B would be encapsulated to Ba when A-B fails, but reachable via A-B would be encapsulated to Ba when A-B fails, but
traffic addressed to any not-via address arriving at A would be traffic addressed to any not-via address arriving at A would be
dropped. The new procedure modifies this such that any traffic for a dropped. The new procedure modifies this such that any traffic for a
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same result could be achieved by checking for multiple levels of same result could be achieved by checking for multiple levels of
encapsulation and dropping any attempt to triple encapsulate. encapsulation and dropping any attempt to triple encapsulate.
However, this would require more detailed inspection of the packet, However, this would require more detailed inspection of the packet,
and causes difficulties when more than 2 "simultaneous" failures are and causes difficulties when more than 2 "simultaneous" failures are
contemplated. contemplated.
So far we have permitted benign repairs to coexist, albeit sometimes So far we have permitted benign repairs to coexist, albeit sometimes
requiring multiple encapsulation. Note that in many cases there will requiring multiple encapsulation. Note that in many cases there will
be no performance impact since unless both failures are on the same be no performance impact since unless both failures are on the same
node, the two encapsulations or two decapsulations will be performed node, the two encapsulations or two decapsulations will be performed
at different nodes. There is however the issue of the MTU impact of at different nodes. There is however the issue of the maximum
multiple encapsulations. transmission unit (MTU) impact of multiple encapsulations.
In the following sub-section we consider the various strategies that In the following sub-section we consider the various strategies that
may be applied to case 3 - mutual repairs that would loop. may be applied to case 3 - mutual repairs that would loop.
6.3.3. Looping Repairs 6.3.3. Looping Repairs
In case 3, the simplest approach is to simply not install repairs for In case 3, the simplest approach is to simply not install repairs for
repair paths that might loop. In this case, although the potentially repair paths that might loop. In this case, although the potentially
looping traffic is dropped, the traffic is not repaired. If we looping traffic is dropped, the traffic is not repaired. If we
assume that a hold-down is applied before reconvergence in case the assume that a hold-down is applied before reconvergence in case the
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[I-D.ietf-rtgwg-ordered-fib]. It is therefore necessary to [I-D.ietf-rtgwg-ordered-fib]. It is therefore necessary to
explicitly trigger an oFIB AAH. explicitly trigger an oFIB AAH.
6.3.3.1. Dropping Looping Packets 6.3.3.1. Dropping Looping Packets
One approach to case 3 is to allow the repair, and to experimentally One approach to case 3 is to allow the repair, and to experimentally
discover the incompatibility of the repairs if and when they occur. discover the incompatibility of the repairs if and when they occur.
With this method we permit the repair in case 3 and trigger AAH when With this method we permit the repair in case 3 and trigger AAH when
a packet drop count on the not-via address has been incremented. a packet drop count on the not-via address has been incremented.
Alternatively, it is possible to wait until the LSP describing the Alternatively, it is possible to wait until the LSP describing the
change is issued normally (i.e. when X announces the failure of X-Y). change is issued normally (i.e. when X announces the failure of
When the repairing node A, which has precomputed that X-Y failures X-Y). When the repairing node A, which has precomputed that X-Y
are mutually incompatible with its own repairs receives this LSP it failures are mutually incompatible with its own repairs receives this
can then issue the AAH. This has the disadvantage that it does not LSP it can then issue the AAH. This has the disadvantage that it
overcome the hold-down delay, but it requires no "data-driven" does not overcome the hold-down delay, but it requires no "data-
operation, and it still has the required effect of abandoning the driven" operation, and it still has the required effect of abandoning
oFIB which is probably the longer of the delays (although with the oFIB which is probably the longer of the delays (although with
signalled oFIB this should be sub-second). signalled oFIB this should be sub-second).
Whilst both of the experimental approaches described above are Whilst both of the experimental approaches described above are
feasible, they tend to induce AAH in the presence of otherwise feasible, they tend to induce AAH in the presence of otherwise
feasible repairs, and they are contrary to the philosophy of repair feasible repairs, and they are contrary to the philosophy of repair
pre-determination that has been applied to existing IPFRR solutions. pre-determination that has been applied to existing IPFRR solutions.
6.3.3.2. Computing non-looping Repairs of Repairs 6.3.3.2. Computing non-looping Repairs of Repairs
An alternative approach to simply dropping the looping packets, or to An alternative approach to simply dropping the looping packets, or to
detecting the loop after it has occurred, is to use secondary SRLGs. detecting the loop after it has occurred, is to use secondary SRLGs.
With a link state routing protocol it is possible to precompute the With a link state routing protocol it is possible to pre-compute the
incompatibility of the repairs in advance and to compute an incompatibility of the repairs in advance and to compute an
alternative SRLG repair path. Although this does considerably alternative SRLG repair path. Although this does considerably
increase the computational complexity it may be possible to compute increase the computational complexity it may be possible to compute
repair paths that avoid the need to simply drop the offending repair paths that avoid the need to simply drop the offending
packets. packets.
This approach requires us to identify the mutually incompatible This approach requires us to identify the mutually incompatible
failures, and advertise them as "secondary SRLGs". When computing failures, and advertise them as "secondary SRLGs". When computing
the repair paths for the affected not-via addresses these links are the repair paths for the affected not-via addresses these links are
simultaneously failed. Note that the assumed simultaneous failure simultaneously failed. Note that the assumed simultaneous failure
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node pairs in common, so the actual number of link failures which node pairs in common, so the actual number of link failures which
require investigation is the union of these sets. It is then require investigation is the union of these sets. It is then
necessary to run an SPF rooted at the first node of each pair (the necessary to run an SPF rooted at the first node of each pair (the
first node because the pairings are ordered representing the first node because the pairings are ordered representing the
direction of the path), with the link to the second node removed. direction of the path), with the link to the second node removed.
This SPF, while not an incremental, can be terminated as soon as the This SPF, while not an incremental, can be terminated as soon as the
not-via address is reached. For example, when running the SPF rooted not-via address is reached. For example, when running the SPF rooted
at X, with the link X-Y removed, the SPF can be terminated when Yx is at X, with the link X-Y removed, the SPF can be terminated when Yx is
reached. Once the path has been found, the path is checked to reached. Once the path has been found, the path is checked to
determine if it traverses any of A's links in the direction away from determine if it traverses any of A's links in the direction away from
A. Note that, because the node pair XY may exist in the list for more A. Note that, because the node pair XY may exist in the list for
than one of A's links (i.e. it lies on more than one repair path), it more than one of A's links (i.e. it lies on more than one repair
is necessary to identify the correct list, and hence link which has a path), it is necessary to identify the correct list, and hence link
mutually looping repair path. That link of A is then advertised by A which has a mutually looping repair path. That link of A is then
as a secondary SRLG paired with the link X-Y. Also note that X will advertised by A as a secondary SRLG paired with the link X-Y. Also
be running this algorithm as well, and will identify that XY is note that X will be running this algorithm as well, and will identify
paired with A-B and so advertise it. This could perhaps be used as a that XY is paired with A-B and so advertise it. This could perhaps
further check. be used as a further check.
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 Appendix A computation. Note also that once we have reached Q-space Appendix A
skipping to change at page 25, line 32 skipping to change at page 24, line 32
the link using a downstream route will have a discard FIB entry for the link using a downstream route will have a discard FIB entry for
the not-via address of the other link. The consequence is that the not-via address of the other link. The consequence is that
potentially looping packets will be discarded when they attempt to potentially looping packets will be discarded when they attempt to
cross this link. cross this link.
In the case where the mutual repairs are both using not-via repairs, In the case where the mutual repairs are both using not-via repairs,
the loop will be broken when the packet arrives at the second the loop will be broken when the packet arrives at the second
failure. However packets are unconditionally repaired by means of a failure. However packets are unconditionally repaired by means of a
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 13) 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.
is not the same topology that was used for the normal downstream This 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
perform the downstream calculation for that not-via address in the must perform the downstream calculation for that not-via address in
topology with link Q-P failed.), then the downstream repair for Yx the topology with link Q-P failed.), then the downstream repair for
can safely be used. These packets cannot re-visit X-Y, since by Yx 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).
7. Optimizing not-via computations using LFAs 7. 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
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A more complete description of multicast operation is for further A more complete description of multicast operation is for further
study. study.
9. Fast Reroute in an MPLS LDP Network. 9. Fast Reroute in an MPLS LDP Network.
Not-via addresses are IP addresses and LDP [RFC5036] will distribute Not-via addresses are IP addresses and LDP [RFC5036] will distribute
labels for them in the usual way. The not-via repair mechanism may labels for them in the usual way. The not-via repair mechanism may
therefore be used to provide fast re-route in an MPLS network by therefore be used to provide fast re-route in an MPLS network by
first pushing the label which the repair endpoint uses to forward the first pushing the label which the repair endpoint uses to forward the
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
Figure 1, if S has a packet destined for D that it must reach via P 1, if S has a packet destined for D that it must reach via P and B, S
and B, S first pushes B's label for D. S then pushes the label that first pushes B's label for D. S then pushes the label that its next
its next hop to Bp needs to reach Bp. 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 a method similar to the neighbor's neighbors, or it needs to use a method similar to the
next-next hop label distribution mechanism proposed in next-next hop label distribution mechanism proposed in
[I-D.shen-mpls-ldp-nnhop-label]. [I-D.shen-mpls-ldp-nnhop-label].
10. Encapsulation 10. Encapsulation
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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.
Care needs to be taken to ensure that the encapsulation used to
provide a repair tunnel does not result in the packet exceeding the
MTU of the links traversed by that repair.
11. Routing Extensions 11. Routing Extensions
IPFRR requires routing protocol extensions. Each IPFRR router that IPFRR requires routing protocol extensions. Each IPFRR router that
is directly connected to a protected network component MUST advertise is directly connected to a protected network component must advertise
a not-via address for that component. This MUST be advertised in a not-via address for that component. This must be advertised in
such a way that the association between the protected component such a way that the association between the protected component
(link, router or SRLG) and the not-via address can be determined by (link, router or SRLG) and the not-via address can be determined by
the other routers 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 7 is to be used the use of If the optimization proposed in Section 7 is to be used, then the use
the LFA in place of the not-via repair MUST also be signalled in the of the LFA in place of the not-via repair MUST also be signalled in
routing protocol. the routing protocol.
12. Incremental Deployment 12. 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.
13. Manageability Considerations 13. Manageability Considerations
[RFC5714] outlines the general set of manageability consideration [RFC5714] outlines the general set of manageability consideration
that apply to the general case of IPFRR. We slightly expand this and that apply to the general case of IPFRR. We slightly expand this and
add details that are not-via specific. There are three classes add details that are not-via specific. There are three classes
manageability consideration: manageability consideration:
1. Pre-failure configuration 1. Pre-failure configuration
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not a well developed IETF technology. not a well developed IETF technology.
14. IANA Considerations 14. IANA Considerations
There are no IANA considerations that arise from this draft. There are no IANA considerations that arise from this draft.
15. Security Considerations 15. 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 [RFC6169]. The primary method of protection SHOULD be
of a private address space for the not-via addresses. These through the use of a private address space for the not-via addresses
addresses MUST NOT be advertised outside the area, and SHOULD be [RFC1918],[RFC4193] . Repair endpoint addresses MUST NOT be
filtered at the network entry points. In addition, a mechanism might advertised outside the area, and MUST be filtered at the network
be developed that allowed the use of the mild security available entry points. In addition, a mechanism might be developed that
through the use of a key [RFC1701] [RFC3931]. With the deployment of allowed the use of the mild security available through the use of a
such mechanisms, the repair endpoints would not increase the security key [RFC1701] [RFC3931]. With the deployment of such mechanisms, the
risk beyond that of existing IP tunnel mechanisms. An attacker may repair endpoints would not increase the security risk beyond that of
attempt to overload a router by addressing an excessive traffic load existing IP tunnel mechanisms. An attacker may attempt to overload a
to the de-capsulation endpoint. Typically, routers take a 50% router by addressing an excessive traffic load to the de-capsulation
performance penalty in decapsulating a packet. The attacker could endpoint. Typically, routers take a 50% performance penalty in
not be certain that the router would be impacted, and the extremely decapsulating a packet. The attacker could not be certain that the
high volume of traffic needed, would easily be detected as an router would be impacted, and the extremely high volume of traffic
anomaly. If an attacker were able to influence the availability of a needed, would easily be detected as an anomaly. If an attacker were
link, they could cause the network to invoke the not-via repair able to influence the availability of a link, they could cause the
mechanism. A network protected by not-via IPFRR is less vulnerable network to invoke the not-via repair mechanism. A network protected
to such an attack than a network that undertook a full convergence in by not-via IPFRR is less vulnerable to such an attack than a network
response to a link up/down event. that undertook a full convergence in response to a link up/down
event.
16. Acknowledgements 16. 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.
17. References 17. References
17.1. Normative References 17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
17.2. Informative References 17.2. Informative References
[I-D.ietf-rtgwg-ordered-fib] [I-D.ietf-rtgwg-ordered-fib]
Shand, M., Bryant, S., Previdi, S., Filsfils, C., Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Loop-free convergence Francois, P., and O. Bonaventure, "Framework for Loop-free
using oFIB", draft-ietf-rtgwg-ordered-fib-07 (work in convergence using oFIB", draft-ietf-rtgwg-ordered-fib-09
progress), September 2012. (work in progress), January 2013.
[I-D.ietf-rtgwg-remote-lfa] [I-D.ietf-rtgwg-remote-lfa]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S. Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-01 Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02
(work in progress), December 2012. (work in progress), May 2013.
[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-
draft-shen-mpls-ldp-nnhop-label-02 (work in progress), 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.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets", BCP
5, RFC 1918, February 1996.
[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.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 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 [RFC5101] Claise, B., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008. 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
RFC 5714, January 2010. 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.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
Appendix A. Q-Space Appendix A. Q-Space
Q-space is the set of routers from which a specific router can be Q-space is the set of routers from which a specific router can be
reached without any path (including equal cost path splits) reached without any path (including equal cost path splits)
transiting the protected link (or node). It is fully described in transiting the protected link (or node). It is fully described in
[I-D.ietf-rtgwg-remote-lfa]. [I-D.ietf-rtgwg-remote-lfa].
S---E S---E
/ \ / \
A D A D
\ / \ /
B---C B---C
Figure 15 Figure 15
Consider a repair of link S-E (Figure 15). The set of routers from 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 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 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 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 shortest path tree (rSPT) rooted at E, with the sub-tree which
traverses the failed link excised (including those which are members traverses the failed link excised (including those which are members
of an ECMP). The rSPT uses the cost towards the root rather than of an ECMP). The rSPT uses the cost towards the root rather than
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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
Stefano Previdi Stefano Previdi
Cisco Systems Cisco Systems
Via Del Serafico, 200 Via Del Serafico, 200
00142 Rome, 00142 Rome
Italy Italy
Email: sprevidi@cisco.com Email: sprevidi@cisco.com
Mike Shand Mike Shand
Individual Contributor Individual Contributor
Email: imc.shand@googlemail.com Email: imc.shand@googlemail.com
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