draft-ietf-rtgwg-ipfrr-notvia-addresses-03.txt   draft-ietf-rtgwg-ipfrr-notvia-addresses-04.txt 
Network Working Group M. Shand Network Working Group M. Shand
Internet-Draft S. Bryant Internet-Draft S. Bryant
Intended status: Standards Track S. Previdi Intended status: Experimental S. Previdi
Expires: May 3, 2009 Cisco Systems Expires: January 11, 2010 Cisco Systems
October 30, 2008 July 10, 2009
IP Fast Reroute Using Not-via Addresses IP Fast Reroute Using Not-via Addresses
draft-ietf-rtgwg-ipfrr-notvia-addresses-03 draft-ietf-rtgwg-ipfrr-notvia-addresses-04
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Abstract Abstract
This draft describes a mechanism that provides fast reroute in an IP This draft describes a mechanism that provides fast reroute in an IP
network through encapsulation to "not-via" addresses. A single level network through encapsulation to "not-via" addresses. A single level
of encapsulation is used. The mechanism protects unicast, multicast of encapsulation is used. The mechanism protects unicast, multicast
and LDP traffic against link, router and shared risk group failure, and LDP traffic against link, router and shared risk group failure,
regardless of network topology and metrics. regardless of network topology and metrics.
Table of Contents Requirements Language
1. Conventions used in this document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 3
3.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . . 5
3.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . . 5
4. Not-via Repair Path Computation . . . . . . . . . . . . . . . 5
4.1. Computing not-via repairs in routing vector protocols . . 6
5. Operation of Repairs . . . . . . . . . . . . . . . . . . . . . 7
5.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 8
5.4. Installation of Repair Paths . . . . . . . . . . . . . . . 9
6. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 11
6.1.1. Use of LFAs with SRLGs . . . . . . . . . . . . . . . . 14
6.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 15
6.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 15
6.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 16
6.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . . 17
7. Multiple Simultaneous Failures . . . . . . . . . . . . . . . . 17
8. Optimizing not-via computations using LFAs . . . . . . . . . . 17
9. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 19
11. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 19
12. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 19
13. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 20
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
15. Security Considerations . . . . . . . . . . . . . . . . . . . 20
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
17.1. Normative References . . . . . . . . . . . . . . . . . . . 21
17.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
1. Conventions used in this document
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 RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Introduction Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Not-via Repairs . . . . . . . . . . . . . . . . . 4
2.1. Use of Equal Cost Multi-Path . . . . . . . . . . . . . . . 6
2.2. Use of LFA repairs . . . . . . . . . . . . . . . . . . . . 6
3. Not-via Repair Path Computation . . . . . . . . . . . . . . . 6
3.1. Computing not-via repairs in routing vector protocols . . 7
4. Operation of Repairs . . . . . . . . . . . . . . . . . . . . . 8
4.1. Node Failure . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Link Failure . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Loop Prevention Under Node Failure . . . . . . . . . . 9
4.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9
4.4. Installation of Repair Paths . . . . . . . . . . . . . . . 10
5. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 12
5.1.1. Use of LFAs with SRLGs . . . . . . . . . . . . . . . . 16
5.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 16
5.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 17
5.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 18
5.2.3. LAN Repair Using Diagnostics . . . . . . . . . . . . . 19
5.3. Multiple Independent Failures . . . . . . . . . . . . . . 19
5.3.1. Looping Repairs . . . . . . . . . . . . . . . . . . . 20
5.3.2. Outline Solution . . . . . . . . . . . . . . . . . . . 21
5.3.3. Looping Repairs . . . . . . . . . . . . . . . . . . . 22
5.3.3.1. Dropping Looping Packets . . . . . . . . . . . . . 22
5.3.3.2. Computing non-looping Repairs of Repairs . . . . . 23
5.3.3.3. N-level Mutual Loops . . . . . . . . . . . . . . . 25
5.3.4. Mixing LFAs and Not-via . . . . . . . . . . . . . . . 25
6. Optimizing not-via computations using LFAs . . . . . . . . . . 26
7. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 27
9. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 28
11. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 28
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
13. Security Considerations . . . . . . . . . . . . . . . . . . . 29
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
15.1. Normative References . . . . . . . . . . . . . . . . . . . 29
15.2. Informative References . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
When a link or a router fails, only the neighbors of the failure are When a link or a router fails, only the neighbors of the failure are
initially aware that the failure has occurred. In a network initially aware that the failure has occurred. In a network
operating IP fast reroute [I-D.ietf-rtgwg-ipfrr-framework], the operating IP fast reroute [I-D.ietf-rtgwg-ipfrr-framework], the
routers that are the neighbors of the failure repair the failure. routers that are the neighbors of the failure repair the failure.
These repairing routers have to steer packets to their destinations These repairing routers have to steer packets to their destinations
despite the fact that most other routers in the network are unaware despite the fact that most other routers in the network are unaware
of the nature and location of the failure. of the nature and location of the failure.
A common limitation in most IPFRR mechanisms is an inability to A common limitation in most IPFRR mechanisms is an inability to
indicate the identity of the failure and to explicitly steer the indicate the identity of the failure and to explicitly steer the
repaired packet round the failure. The extent to which this repaired packet round the failure. The extent to which this
limitation affects the repair coverage is topology dependent. The limitation affects the repair coverage is topology dependent. The
mechanism proposed here is to encapsulate the packet to an address mechanism proposed here is to encapsulate the packet to an address
that explicitly identifies the network component that the repair must that explicitly identifies the network component that the repair must
avoid. This produces a repair mechanism, which, provided the network avoid. This produces a repair mechanism, which, provided the network
is not partitioned by the failure, will always achieve a repair. is not partitioned by the failure, will always achieve a repair.
3. Overview of Not-via Repairs 2. Overview of Not-via Repairs
When a link or a router fails, only the neighbors of the failure are When a link or a router fails, only the neighbors of the failure are
initially aware that the failure has occurred. In a network initially aware that the failure has occurred. In a network
operating IP fast reroute [I-D.ietf-rtgwg-ipfrr-framework], the operating IP fast reroute [I-D.ietf-rtgwg-ipfrr-framework], the
routers that are the neighbors of the failure repair the failure. routers that are the neighbors of the failure repair the failure.
These repairing routers have to steer packets to their destinations These repairing routers have to steer packets to their destinations
despite the fact that most other routers in the network are unaware despite the fact that most other routers in the network are unaware
of the nature and location of the failure. of the nature and location of the failure.
A common limitation in most IPFRR mechanisms is an inability to A common limitation in most IPFRR mechanisms is an inability to
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| |
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 2.1. Use of Equal Cost Multi-Path
A router can use an equal cost multi-path (ECMP) repair in place of a A router can use an equal cost multi-path (ECMP) repair in place of a
not-via repair. not-via repair.
A router computing a not-via repair path MAY subject the repair to A router computing a not-via repair path MAY subject the repair to
ECMP. ECMP.
3.2. Use of LFA repairs 2.2. Use of LFA repairs
The not-via approach provides complete repair coverage and therefore The not-via approach provides complete repair coverage and therefore
may be used as the sole repair mechanism. There are, however, may be used as the sole repair mechanism. There are, however,
advantages in using not-via in combination with loop free alternates advantages in using not-via in combination with loop free alternates
(LFA) and or downstream paths as documented in [RFC5286]. (LFA) and or downstream paths as documented in [RFC5286].
LFAs are computed on a per destination basis and in general, only a LFAs are computed on a per destination basis and in general, only a
subset of the destinations requiring repair will have a suitable LFA subset of the destinations requiring repair will have a suitable LFA
repair. In this case, those destinations which are repairable by repair. In this case, those destinations which are repairable by
LFAs are so repaired and the remainder of the destinations are LFAs are so repaired and the remainder of the destinations are
repaired using the not-via encapsulation. This has the advantage of repaired using the not-via encapsulation. This has the advantage of
reducing the volume of traffic that requires encapsulation. On the reducing the volume of traffic that requires encapsulation. On the
other hand, the path taken by an LFA repair may be less optimal than other hand, the path taken by an LFA repair may be less optimal than
that of the equivalent not-via repair for traffic destined to nodes that of the equivalent not-via repair for traffic destined to nodes
close to the far end of the failure, but may be more optimal for some close to the far end of the failure, but may be more optimal for some
other traffic. The description in this document assumes that LFAs other traffic. The description in this document assumes that LFAs
will be used where available, but the distribution of repairs between will be used where available, but the distribution of repairs between
the two mechanisms is a local implementation choice. the two mechanisms is a local implementation choice.
4. Not-via Repair Path Computation 3. Not-via Repair Path Computation
The not-via repair mechanism requires that all routers on the path The not-via repair mechanism requires that all routers on the path
from S to B (Figure 1) have a route to Bp. They can calculate this from S to B (Figure 1) have a route to Bp. They can calculate this
by failing node P, running an SPF, and finding the shortest route to by failing node P, running an SPF, and finding the shortest route to
B. B.
A router has no simple way of knowing whether it is on the shortest A router has no simple way of knowing whether it is on the shortest
path for any particular repair. It is therefore necessary for every path for any particular repair. It is therefore necessary for every
router to calculate the path it would use in the event of any router to calculate the path it would use in the event of any
possible router failure. Each router therefore "fails" every router possible router failure. Each router therefore "fails" every router
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This algorithm is significantly less expensive than a set of full This algorithm is significantly less expensive than a set of full
SPFs. Thus, although a router has to calculate the repair paths for SPFs. Thus, although a router has to calculate the repair paths for
n-1 failures, the computational effort is much less than n-1 SPFs. n-1 failures, the computational effort is much less than n-1 SPFs.
Experiments on a selection of real world network topologies with Experiments on a selection of real world network topologies with
between 40 and 400 nodes suggest that the worst-case computational between 40 and 400 nodes suggest that the worst-case computational
complexity using the above optimizations is equivalent to performing complexity using the above optimizations is equivalent to performing
between 5 and 13 full SPFs. Further optimizations are described in between 5 and 13 full SPFs. Further optimizations are described in
section 6. section 6.
4.1. Computing not-via repairs in routing vector protocols 3.1. Computing not-via repairs in routing vector protocols
While this document focuses on link state routing protocols, it is While this document focuses on link state routing protocols, it is
equally possible to compute not-via repairs in distance vector (e.g. equally possible to compute not-via repairs in distance vector (e.g.
RIP) or path vector (e.g. BGP) routing protocols. This can be RIP) or path vector (e.g. BGP) routing protocols. This can be
achieved with very little protocol modification by advertising the achieved with very little protocol modification by advertising the
not-via address in the normal way, but ensuring that the information not-via address in the normal way, but ensuring that the information
about a not-via address Ps is not propagated through the node S. In about a not-via address Ps is not propagated through the node S. In
the case of link protection this simply means that the advertisement the case of link protection this simply means that the advertisement
from P to S is suppressed, with the result that S and all other nodes from P to S is suppressed, with the result that S and all other nodes
compute a route to Ps which doesn't traverse S, as required. compute a route to Ps which doesn't traverse S, as required.
In the case of node protection, where P is the protected node, and N In the case of node protection, where P is the protected node, and N
is some neighbor, the advertisement of Np must be suppressed not only is some neighbor, the advertisement of Np must be suppressed not only
across the link N->P, but also across any link to P. The simplest way across the link N->P, but also across any link to P. The simplest way
of achieving this is for P itself to perform the suppression of any of achieving this is for P itself to perform the suppression of any
address of the form Xp. address of the form Xp.
5. Operation of Repairs 4. Operation of Repairs
This section explains the basic operation of the not-via repair of This section explains the basic operation of the not-via repair of
node and link failure. node and link failure.
5.1. Node Failure 4.1. Node Failure
When router P fails (Figure 2) S encapsulates any packet that it When router P fails (Figure 2) S encapsulates any packet that it
would send to B via P to Bp, and then sends the encapsulated packet would send to B via P to Bp, and then sends the encapsulated packet
on the shortest path to Bp. S follows the same procedure for routers on the shortest path to Bp. S follows the same procedure for routers
A and C in Figure 2. The packet is decapsulated at the repair target A and C in Figure 2. The packet is decapsulated at the repair target
(A, B or C) and then forwarded normally to its destination. The (A, B or C) and then forwarded normally to its destination. The
repair target can be determined as part of the normal SPF by repair target can be determined as part of the normal SPF by
recording the "next-next-hop" for each destination in addition to the recording the "next-next-hop" for each destination in addition to the
normal next-hop. normal next-hop.
Notice that with this technique only one level of encapsulation is Notice that with this technique only one level of encapsulation is
needed, and that it is possible to repair ANY failure regardless of needed, and that it is possible to repair ANY failure regardless of
link metrics and any asymmetry that may be present in the network. link metrics and any asymmetry that may be present in the network.
The only exception to this is where the failure was a single point of The only exception to this is where the failure was a single point of
failure that partitioned the network, in which case ANY repair is failure that partitioned the network, in which case ANY repair is
clearly impossible. clearly impossible.
5.2. Link Failure 4.2. Link Failure
The normal mode of operation of the network would be to assume router The normal mode of operation of the network would be to assume router
failure. However, where some destinations are only reachable through failure. However, where some destinations are only reachable through
the failed router, it is desirable that an attempt be made to repair the failed router, it is desirable that an attempt be made to repair
to those destinations by assuming that only a link failure has to those destinations by assuming that only a link failure has
occurred. occurred.
To perform a link repair, S encapsulates to Ps (i.e. it instructs the To perform a link repair, S encapsulates to Ps (i.e. it instructs the
network to deliver the packet to P not-via S). All of the neighbors network to deliver the packet to P not-via S). All of the neighbors
of S will have calculated a path to Ps in case S itself had failed. of S will have calculated a path to Ps in case S itself had failed.
S could therefore give the packet to any of its neighbors (except, of S could therefore give the packet to any of its neighbors (except, of
course, P). However, S should preferably send the encapsulated course, P). However, S should preferably send the encapsulated
packet on the shortest available path to P. This path is calculated packet on the shortest available path to P. This path is calculated
by running an SPF with the link SP failed. Note that this may again by running an SPF with the link SP failed. Note that this may again
be an incremental calculation, which can terminate when address Ps be an incremental calculation, which can terminate when address Ps
has been reattached. has been reattached.
4.2.1. Loop Prevention Under Node Failure
It is necessary to consider the behavior of IPFRR solutions when a It is necessary to consider the behavior of IPFRR solutions when a
link repair is attempted in the presence of node failure. In its link repair is attempted in the presence of node failure. In its
simplest form the not-via IPFRR solution prevents the formation of simplest form the not-via IPFRR solution prevents the formation of
loops forming as a result of mutual repair, by never providing a loops forming as a result of mutual repair, by never providing a
repair path for a not-via address. Referring to Figure 2, if A was repair path for a not-via address. The repair of packets with not-
the neighbor of P that was on the link repair path from S to P, and P via addresses is considered in more detail in Section 5.3. Referring
itself had failed, the repaired packet from S would arrive at A to Figure 2, if A was the neighbor of P that was on the link repair
encapsulated to Ps. A would have detected that the AP link had path from S to P, and P itself had failed, the repaired packet from S
failed and would normally attempt to repair the packet. However, no would arrive at A encapsulated to Ps. A would have detected that the
repair path is provided for any not-via address, and so A would be AP link had failed and would normally attempt to repair the packet.
forced to drop the packet, thus preventing the formation of loop. However, no repair path is provided for any not-via address, and so A
would be forced to drop the packet, thus preventing the formation of
loop.
5.3. Multi-homed Prefixes 4.3. Multi-homed Prefixes
A multi-homed Prefix (MHP) is a prefix that is reachable via more A multi-homed Prefix (MHP) is a prefix that is reachable via more
than one router in the network. Some of these may be repairable than one router in the network. Some of these may be repairable
using LFAs as described in [RFC5286]. Only those without such a using LFAs as described in [RFC5286]. Only those without such a
repair need be considered here. repair need be considered here.
When IPFRR router S (Figure 3) discovers that P has failed, it needs When IPFRR router S (Figure 3) discovers that P has failed, it needs
to send packets addressed to the MHP X, which is normally reachable to send packets addressed to the MHP X, which is normally reachable
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.
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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 4.4. Installation of Repair Paths
The following algorithm is used by node S (Figure 3) to pre- The following algorithm is used by node S (Figure 3) to pre-
calculate and install repair paths in the FIB, ready for immediate calculate and install repair paths in the FIB, ready for immediate
use in the event of a failure. It is assumed that the not-via repair use in the event of a failure. It is assumed that the not-via repair
paths have already been calculated as described above. paths have already been calculated as 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
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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.
6. Compound Failures 5. Compound Failures
The following types of failures involve more tha one component. The following types of failures involve more than one component:
6.1. Shared Risk Link Groups 1. Shared Risk Link Groups
2. Local Area Networks
3. Multiple Independent Failures
The considerations that apply in each of the above situations are
described in the following sections.
5.1. Shared Risk Link Groups
A Shared Risk Link Group (SRLG) is a set of links whose failure can A Shared Risk Link Group (SRLG) is a set of links whose failure can
be caused by a single action such as a conduit cut or line card be caused by a single action such as a conduit cut or line card
failure. When repairing the failure of a link that is a member of an failure. When repairing the failure of a link that is a member of an
SRLG, it must be assumed that all the other links that are also SRLG, it must be assumed that all the other links that are also
members of the SRLG have also failed. Consequently, any repair path members of the SRLG have also failed. Consequently, any repair path
must be computed to avoid not just the adjacent link, but also all must be computed to avoid not just the adjacent link, but also all
the links which are members of the same SRLG. the links which are members of the same SRLG.
In Figure 4 below, the links S-P and A-B are both members of SRLG In Figure 4 below, the links S-P and A-B are both members of SRLG
skipping to change at page 12, line 23 skipping to change at page 13, line 41
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 de-capsulated before the packet is re-encapsulated using the not- is decapsulated before the packet is re-encapsulated using the not-
via address corresponding to the far side of the next link which is a via address corresponding to the far side of the next link which is a
member of the same SRLG. In some cases the de-capsulation and re- member of the same SRLG. In some cases the de-capsulation 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 | | |
skipping to change at page 12, line 50 skipping to change at page 14, line 25
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 SRLG 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
skipping to change at page 14, line 46 skipping to change at page 16, line 24
2. Modifying the design of the network to avoid this possibility. 2. Modifying the design of the network to avoid this possibility.
3. Using some form of SRLG diagnostic (for example, by running BFD 3. Using some form of SRLG diagnostic (for example, by running BFD
over alternate repair paths) to determine which SRLG member(s) over alternate repair paths) to determine which SRLG member(s)
has actually failed and using this information to select an has actually failed and using this information to select an
appropriate pre-computed repair path. However, aside from the appropriate pre-computed repair path. However, aside from the
complexity of performing the diagnostics, this requires multiple complexity of performing the diagnostics, this requires multiple
not-via addresses per interface, which has poor scaling not-via addresses per interface, which has poor scaling
properties. properties.
6.1.1. Use of LFAs with SRLGs 4. Using the machanism described in Section 5.3
5.1.1. Use of LFAs with SRLGs
Section 4.1 above describes the repair of links which are members of Section 4.1 above describes the repair of links which are members of
one or more SRLGs. LFAs can be used for the repair of such links one or more SRLGs. LFAs can be used for the repair of such links
provided that any other link with which S-P shares an SRLG is avoided provided that any other link with which S-P shares an SRLG is avoided
when computing the LFA. This is described for the simple case of when computing the LFA. This is described for the simple case of
"local-SRLGs" in [RFC5286]. "local-SRLGs" in [RFC5286].
6.2. Local Area Networks 5.2. Local Area Networks
LANs are a special type of SRLG and are solved using the SRLG LANs are a special type of SRLG and are solved using the SRLG
mechanisms outlined above. With all SRLGs there is a trade-off mechanisms outlined above. With all SRLGs there is a trade-off
between the sophistication of the fault detection and the size of the between the sophistication of the fault detection and the size of the
SRLG. Protecting against link failure of the LAN link(s) is SRLG. Protecting against link failure of the LAN link(s) is
relatively straightforward, but as with all fast reroute mechanisms, relatively straightforward, but as with all fast reroute mechanisms,
the problem becomes more complex when it is desired to protect the problem becomes more complex when it is desired to protect
against the possibility of failure of the nodes attached to the LAN against the possibility of failure of the nodes attached to the LAN
as well as the LAN itself. as well as the LAN itself.
skipping to change at page 15, line 46 skipping to change at page 17, line 34
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 5.2.1. 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
| |
skipping to change at page 16, line 30 skipping to change at page 18, line 30
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 would
be addressed to P not-via the LAN or any router attached to the LAN be addressed to P not-via the LAN or any router attached to the LAN
(except of course P). (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 5.2.2. LAN Component Repair
In this approach, possible failures are considered at a finer In this approach, possible failures are considered at a finer
granularity, but without the use of diagnostics to identify the granularity, but without the use of diagnostics to identify the
specific component that has failed. Because S is unable to diagnose specific component that has failed. Because S is unable to diagnose
the failure it must repair traffic sent through P and B, to B not- the failure it must repair traffic sent through P and B, to B not-
via P,N (i.e. not via P and not via N), on the conservative via P,N (i.e. not via P and not via N), on the conservative
assumption that both the entire LAN and P have failed. Destinations assumption that both the entire LAN and P have failed. Destinations
for which P is a single point of failure must as usual be sent to P for which P is a single point of failure must as usual be sent to P
using an address that avoids the interface by which P is reached from using an address that avoids the interface by which P is reached from
S, i.e. to P not-via N. Similarly for routers Q and R. S, i.e. to P not-via N. Similarly for routers Q and R.
skipping to change at page 17, line 21 skipping to change at page 19, line 21
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 5.2.3. 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.
7. Multiple Simultaneous Failures 5.3. Multiple Independent Failures
The failure of a node or an SRLG can result in multiple correlated IPFRR repair of multiple simultaneous failures which are not members
failures, which may be repaired using the mechanisms described in of a known SRLG is complicated by the problem that the use of
this design. This design will not correctly repair a set of multiple concurrent repairs may result in looping repair paths. As
unanticipated multiple failures. Such failures are out of scope of described in Section 4.2.1, the simplest method of preventing such
this design and are for further study. loops, is to ensure that packets addressed to a not-via address are
not repaired but instead are dropped. It is possible that a network
may experience multiple simultaneous failures. This may be due to
simple statistical effects, but the more likely cause is
unanticipated SRLGs. When multiple failures which are not part of an
anticipated group are detected, repairs are abandoned and the network
reverts to normal convergence. Although safe, this approach is
somewhat draconian, since there are many circumstances were multiple
repairs do not induce loops.
It is important that the routers in the network are able to This section describes the properties of multiple unrelated failures
discriminate between these two classes of failure, and take and proposes some methods that may be used to address this problem.
appropriate action.
8. Optimizing not-via computations using LFAs 5.3.1. Looping Repairs
Let us assume that the repair mechanism is based on solely on not-via
repairs. LFA or downstream routes may be incorporated, and will be
dealt with later.
A------//------B------------D
/ \
/ \
F G
\ /
\ /
X------//------Y
Figure 13: The General Case of Multiple Failures
The essential case is as illustrated in Figure 13. Note that
depending on the repair case under consideration, there may be paths
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
between X and Y. These paths are omitted for graphical clarity.
There are three cases to consider:
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
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
not cause looping or packet loss.
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
packet from A to D traverses two concatenated repairs.
A------//------B------------X------//------Y------D
| | | |
| | | |
M--------------+ N--------------+
Figure 14: Concatenated Repairs
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
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
multi-failure mechanism described in this section the repaired
packet for A-B would be dropped when it reached X-Y, since the
repair of repaired packets would be forbidden. However, if this
packet were allowed to be repaired, the path to D would be
complete and no harm would be done, although two levels of
encapsulation would be required.
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
looping packets and increasing levels of encapsulation.
The challenge in applying IPFRR to a network that is undergoing
multiple failures is, therefore, to identify which of these cases
exist in the network and react accordingly.
5.3.2. Outline Solution
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
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
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
would be: Fa, Xf, Yx, Gy, Bg. Under standard not-via operation, A
would populate its FIB such that all normal addresses normally
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
dropped. The new procedure modifies this such that any traffic for a
not-via address normally reachable over A-B is also encapsulated to
Ba unless the not-via address is one of those previously identified
as being on the path to Ba, for example Yx, in which case the packet
is dropped.
The above procedure allows cases 1 and 2 above to be repaired, while
preventing the loop which would result from case 3.
Note that this is accomplished by pre-computing the required FIB
entries, and does not require any detailed packet inspection. The
same result could be achieved by checking for multiple levels of
encapsulation and dropping any attempt to triple encapsulate.
However, this would require more detailed inspection of the packet,
and causes difficulties when more than 2 "simultaneous" failures are
contemplated.
So far we have permitted benign repairs to coexist, albeit sometimes
requiring multiple encapsulation. Note that in many cases there will
be no performance impact since unless both failures are on the same
node, the two encapsulations or two decapsulations will be performed
at different nodes. There is however the issue of the MTU impact of
multiple encapsulations.
In the following sub-section we consider the various strategies that
may be applied to case 3 - mutual repairs that would loop.
5.3.3. Looping Repairs
In case 3, the simplest approach is to simply not install repairs for
repair paths that might loop. In this case, although the potentially
looping traffic is dropped, the traffic is not repaired. If we
assume that a hold-down is applied before reconvergence in case the
link failure was just a short glitch, and if a loop free convergence
mechanism further delays convergence, then the traffic will be
dropped for an extended period. In these circumstances it would be
better to "abandon all hope" (AAH)
[<draft-bryant-francois-shand-ipfrr-aah-00.txt>] and immediately
invoke normal re-convergence.
Note that it is not sufficient to expedite the issuance of an LSP
reporting the failure, since this may be treated as a permitted
simultaneous failure by the oFIB algorithm. It is therefore
necessary to explicitly trigger an oFIB AAH.
5.3.3.1. Dropping Looping Packets
One approach to case 3 is to allow the repair, and to experimentally
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
a packet drop count on the not-via address has been incremented.
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).
When the repairing node A, which has precomputed that X-Y failures
are mutually incompatible with its own repairs receives this LSP it
can then issue the AAH. This has the disadvantage that it doesn't
overcome the hold-down delay, but it requires no "data-driven"
operation, and it still has the required effect of abandoning the
oFIB which is probably the longer of the delays (although with
signalled oFIB this should be sub-second).
Whilst both of the experimental approaches described above are
feasible, they tend to induce AAH in the presence of otherwise
feasible repairs, and they are contrary to the philosophy of repair
pre-determination that has been applied to existing IPFRR solutions.
5.3.3.2. Computing non-looping Repairs of Repairs
An alternative approach to simply dropping the looping packets, or to
detecting the loop after it has occurred, is to use secondary SRLGs.
With a link state routing protocol it is possible to precompute the
incompatibility of the repairs in advance and to compute an
alternative SRLG repair path. Although this does considerably
increase the computational complexity it may be possible to compute
repair paths that avoid the need to simply drop the offending
packets.
This approach requires us to identify the mutually incompatible
failures, and advertise them as "secondary SRLGs". When computing
the repair paths for the affected not-via addresses these links are
simultaneously failed. Note that the assumed simultaneous failure
and resulting repair path only applies to the repair path computed
for the conflicting not-via addresses, and is not used for normal
addresses. This implies that although there will be a longer repair
path when there is more than one failure, if there is a single
failure the repair path length will be "normal".
Ideally we would wish to only invoke secondary SRLG computation when
we are sure that the repair paths are mutually incompatible.
Consider the case of node A in Figure 13. A first identifies that
the repair path for A-B is via F-X-Y-G-B. It then explores this path
determining the repair path for each link in the path. Thus, for
example, it performs a check at X by running an SPF rooted at X with
the X-Y link removed to determine whether A-B is indeed on X's repair
path for packets addressed to Yx.
Some optimizations are possible in this calculation, which appears at
first sight to be order hk (where h is the average hop length of
repair paths and k is the average number of neighbours of a router).
When A is computing its set of repair paths, it does so for all its k
neighbours. In each case it identifies a list of node pairs
traversed by each repair. These lists may often have one or more
node pairs in common, so the actual number of link failures which
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
first node because the pairings are ordered representing the
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
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
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
A. Note that, because the node pair XY may exist in the list for more
than one of A's links (i.e. it lies on more than one repair path), it
is necessary to identify the correct list, and hence link which has a
mutually looping repair path. That link of A is then advertised by A
as a secondary SRLG paired with the link X-Y. Also note that X will
be running this algorithm as well, and will identify that XY is
paired with A-B and so advertise it. This could perhaps be used as a
further check.
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
mutually incompatible, we need to advertise the pair of links as a
secondary SRLG, and then ALL nodes compute repair paths around both
failures using an additional not-via address with the semantics not-
via A-B AND not-via X-Y.
A further possibility is that because we are going to the trouble of
advertising these SRLG sets, we could also advertise the new repair
path and only get the nodes on that path to perform the necessary
computation. Note also that once we have reached Q space with
respect to the two failures we need no longer continue the
computation, so we only need to notify the nodes on the path that are
not in Q-space.
One cause of mutually looping repair paths is the existence of nodes
with only two links, or sections of the network which are only bi-
connected. In these cases, repair is clearly impossible - the
failure of both links partitions the network. It would be
advantageous to be able to identify these cases, and inhibit the
fruitless advertisement of the secondary SRLG information. This
could be achieved by the node detecting the requirement for a
secondary SRLG, first running the not-via computation with both links
removed. If this does not result in a path, it is clear that the
network would be partitioned by such a failure, and so no
advertisement is required.
5.3.3.3. N-level Mutual Loops
[Editors' Note: This section needs to be reviewed before final
publication]
It is tempting to conclude that the mechanism described above can be
applied to the general case of N failures. If we use the approach of
assuming that repairs are not mutual, and catching the loops and
executing AAH when they occur, then we can attempt repairs in the
case of N failures.
If we use the approach of avoiding potentially mutual repairs and
creating secondary SRLG, then we have to explore N levels of repair,
where N is the number of simultaneous failures we wish to repair.
5.3.4. Mixing LFAs and Not-via
So far in this section we have assumed that all repairs use not-via
tunnels. However, in practise we may wish to use LFAs or downstream
routes where available. This complicates the issue, because their
use results in packets which are being repaired, but NOT addressed to
not-via addresses. If BOTH links are using downstream routes there
is no possibility of looping, since it is impossible to have a pair
of nodes which are both downstream of each other [RFC5286].
Loops can however occur when LFAs are used. An obvious example is
the well known node repair problem with LFAs [RFC5286]. If one link
is using a downstream route, while the other is using a not-via
tunnel, the potential mechanism described above would work provided
it were possible to determine the nodes on the path of the downstream
route. Some methods of computing downstream routes do not provide
this path information. If the path information is however available,
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
potentially looping packets will be discarded when they attempt to
cross this link.
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
failure. However packets are unconditionally repaired by means 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
be dropped when it gets back to the first failure. i.e. it will
execute a single turn of the loop before being dropped.
There is a further complication with downstream routes, since
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
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
A-B repair (Figure 1) is a downstream route and the X-Y repair is a
not-via repair, we can have the situation where the X-Y repair
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
route, it cannot be assumed that the repair path for packets
addressed to Yx can be sent to the same neighbour. This is because
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
is not the same topology that was used for the normal downstream
calculation, and use of the normal downstream route for the
encapsulated packets may result in an undetected loop. If it is
computationally feasible to check the downstream route in this
topology (i.e. for any not-via address Qp which traverses A-B we must
perform the downstream calculation for that not-via address in the
topology with link Q-P failed.), then the downstream repair for Yx
can safely be used. These packets cannot re-visit X-Y, since by
definition they will avoid that link. Alternatively, the packet
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
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
installed for an address Qp if it were established that it did not
traverse Q-P (using the rules described above).
6. Optimizing not-via computations using LFAs
If repairing node S has an LFA to the repair endpoint it is not If repairing node S has an LFA to the repair endpoint it is not
necessary for any router to perform the incremental SPF with the link necessary for any router to perform the incremental SPF with the link
SP removed in order to compute the route to the not-via address Ps. SP removed in order to compute the route to the not-via address Ps.
This is because the correct routes will already have been computed as This is because the correct routes will already have been computed as
a result of the SPF on the base topology. Node S can signal this a result of the SPF on the base topology. Node S can signal this
condition to all other routers by including a bit in its LSP or LSA condition to all other routers by including a bit in its LSP or LSA
associated with each LFA protected link. Routers computing not-via associated with each LFA protected link. Routers computing not-via
routes can then omit the running of the iSPF for links with this bit routes can then omit the running of the iSPF for links with this bit
set. set.
skipping to change at page 18, line 36 skipping to change at page 27, line 18
of the protocol is not compromised provided that the necessity to of the protocol is not compromised provided that the necessity to
perform a not-via computation is re-evaluated whenever new perform a not-via computation is re-evaluated whenever new
information arrives. information arrives.
This optimization is not particularly beneficial to nodes close to This optimization is not particularly beneficial to nodes close to
the repair since, as has been observed above, the computation for the repair since, as has been observed above, the computation for
nodes on the LFA path is trivial. However, for nodes upstream of the nodes on the LFA path is trivial. However, for nodes upstream of the
link SP for which S-P is in the path to P, there is a significant link SP for which S-P is in the path to P, there is a significant
reduction in the computation required. reduction in the computation required.
9. Multicast 7. Multicast
Multicast traffic can be repaired in a similar way to unicast. The Multicast traffic can be repaired in a similar way to unicast. The
multicast forwarder is able to use the not-via address to which the multicast forwarder is able to use the not-via address to which the
multicast packet was addressed as an indication of the expected multicast packet was addressed as an indication of the expected
receive interface and hence to correctly run the required RPF check. receive interface and hence to correctly run the required RPF check.
In some cases, all the destinations, including the repair endpoint, In some cases, all the destinations, including the repair endpoint,
are repairable by an LFA. In this case, all unicast traffic may be are repairable by an LFA. In this case, all unicast traffic may be
repaired without encapsulation. Multicast traffic still requires repaired without encapsulation. Multicast traffic still requires
encapsulation, but for the nodes on the LFA repair path the encapsulation, but for the nodes on the LFA repair path the
computation of the not-via forwarding entry is unnecessary since, by computation of the not-via forwarding entry is unnecessary since, by
definition, their normal path to the repair endpoint is not via the definition, their normal path to the repair endpoint is not via the
failure. failure.
A more complete description of multicast operation will be provided A more complete description of multicast operation is for further
in a future version of this draft. study.
10. Fast Reroute in an MPLS LDP Network. 8. 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 1, if S has a packet destined for D that it must reach via P Figure 1, if S has a packet destined for D that it must reach via P
and B, S first pushes B's label for D. S then pushes the label that and B, S first pushes B's label for D. S then pushes the label that
its next hop to Bp needs to reach Bp. its next hop to Bp needs to reach Bp.
Note that in an MPLS LDP network it is necessary for S to have the Note that in an MPLS LDP network it is necessary for S to have the
repair endpoint's label for the destination. When S is effecting a repair endpoint's label for the destination. When S is effecting a
link repair it already has this. In the case of a node repair, S link repair it already has this. In the case of a node repair, S
either needs to set up a directed LDP session with each of its either needs to set up a directed LDP session with each of its
neighbor's neighbors, or it needs to use the next-next hop label neighbor's neighbors, or it needs to use the next-next hop label
distribution mechanism proposed in [I-D.shen-mpls-ldp-nnhop-label]. distribution mechanism proposed in [I-D.shen-mpls-ldp-nnhop-label].
11. Encapsulation 9. Encapsulation
Any IETF specified IP in IP encapsulation may be used to carry a not- Any IETF specified IP in IP encapsulation may be used to carry a not-
via repair. IP in IP [RFC2003], GRE [RFC1701] and L2TPv3 [RFC3931], via repair. IP in IP [RFC2003], GRE [RFC1701] and L2TPv3 [RFC3931],
all have the necessary and sufficient properties. The requirement is all have the necessary and sufficient properties. The requirement is
that both the encapsulating router and the router to which the that both the encapsulating router and the router to which the
encapsulated packet is addressed have a common ability to process the encapsulated packet is addressed have a common ability to process the
chosen encapsulation type. When an MPLS LDP network is being chosen encapsulation type. When an MPLS LDP network is being
protected, the encapsulation would normally be an additional MPLS protected, the encapsulation would normally be an additional MPLS
label. In an MPLS enabled IP network an MPLS label may be used in label. In an MPLS enabled IP network an MPLS label may be used in
place of an IP in IP encapsulation in the case above. place of an IP in IP encapsulation in the case above.
12. Routing Extensions 10. Routing Extensions
IPFRR requires IGP extensions. Each IPFRR router that is directly IPFRR requires IGP extensions. Each IPFRR router that is directly
connected to a protected network component must advertise a not-via connected to a protected network component must advertise a not-via
address for that component. This must be advertised in such a way address for that component. This must be advertised in such a way
that the association between the protected component (link, router or that the association between the protected component (link, router or
SRLG) and the not-via address can be determined by the other routers SRLG) and the not-via address can be determined by the other routers
in the network. 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.
13. Incremental Deployment 11. Incremental Deployment
Incremental deployment is supported by excluding routers that are not Incremental deployment is supported by excluding routers that are not
calculating not-via routes (as indicated by their capability calculating not-via routes (as indicated by their capability
information flooded with their link state information) from the base information flooded with their link state information) from the base
topology used for the computation of repair paths. In that way topology used for the computation of repair paths. In that way
repairs may be steered around islands of routers that are not IPFRR repairs may be steered around islands of routers that are not IPFRR
capable. Routers that are protecting a network component need to capable. Routers that are protecting a network component need to
have the capability to encapsulate and de-capsulate packets. have the capability to encapsulate and decapsulate packets. However,
However, routers that are on the repair path only need to be capable routers that are on the repair path only need to be capable of
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.
14. IANA Considerations 12. 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 13. Security Considerations
The repair endpoints present vulnerability in that they might be used The repair endpoints present vulnerability in that they might be used
as a method of disguising the delivery of a packet to a point in the as a method of disguising the delivery of a packet to a point in the
network. The primary method of protection should be through the use network. The primary method of protection should be through the use
of a private address space for the not-via addresses. These of a private address space for the not-via addresses. These
addresses MUST NOT be advertised outside the area, and SHOULD be addresses MUST NOT be advertised outside the area, and SHOULD be
filtered at the network entry points. In addition, a mechanism might filtered at the network entry points. In addition, a mechanism might
be developed that allowed the use of the mild security available be developed that allowed the use of the mild security available
through the use of a key [RFC1701] [RFC3931]. With the deployment of through the use of a key [RFC1701] [RFC3931]. With the deployment of
such mechanisms, the repair endpoints would not increase the security such mechanisms, the repair endpoints would not increase the security
skipping to change at page 21, line 5 skipping to change at page 29, line 33
to the de-capsulation endpoint. Typically, routers take a 50% to the de-capsulation endpoint. Typically, routers take a 50%
performance penalty in decapsulating a packet. The attacker could performance penalty in decapsulating a packet. The attacker could
not be certain that the router would be impacted, and the extremely not be certain that the router would be impacted, and the extremely
high volume of traffic needed, would easily be detected as an high volume of traffic needed, would easily be detected as an
anomaly. If an attacker were able to influence the availability of a anomaly. If an attacker were able to influence the availability of a
link, they could cause the network to invoke the not-via repair link, they could cause the network to invoke the not-via repair
mechanism. A network protected by not-via IPFRR is less vulnerable mechanism. A network protected by not-via IPFRR is less vulnerable
to such an attack than a network that undertook a full convergence in to such an attack than a network that undertook a full convergence in
response to a link up/down event. response to a link up/down event.
16. Acknowledgements 14. 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 15. References
17.1. Normative References 15.1. 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 15.2. Informative References
[I-D.ietf-bfd-base] [I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-08 (work in progress), Detection", draft-ietf-bfd-base-09 (work in progress),
March 2008. February 2009.
[I-D.ietf-rtgwg-ipfrr-framework] [I-D.ietf-rtgwg-ipfrr-framework]
Shand, M. and S. Bryant, "IP Fast Reroute Framework", Shand, M. and S. Bryant, "IP Fast Reroute Framework",
draft-ietf-rtgwg-ipfrr-framework-08 (work in progress), draft-ietf-rtgwg-ipfrr-framework-11 (work in progress),
February 2008. June 2009.
[I-D.shen-mpls-ldp-nnhop-label] [I-D.shen-mpls-ldp-nnhop-label]
Shen, N., "Discovering LDP Next-Nexthop Labels", Shen, N., "Discovering LDP Next-Nexthop Labels",
draft-shen-mpls-ldp-nnhop-label-02 (work in progress), draft-shen-mpls-ldp-nnhop-label-02 (work in progress),
May 2005. May 2005.
[ISPF] McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing [ISPF] McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing
Algorithm Improvements"", BBN Technical Report 3803, 1978. Algorithm Improvements"", BBN Technical Report 3803, 1978.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
skipping to change at page 23, line 4 skipping to change at line 1314
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
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