< draft-ietf-rtgwg-ipfrr-notvia-addresses-00.txt   draft-ietf-rtgwg-ipfrr-notvia-addresses-01.txt >
INTERNET DRAFT IP Fast Reroute Using Not-via Addresses Dec 2006
Network Working Group S. Bryant Network Working Group S. Bryant
Internet Draft M. Shand Internet Draft M. Shand
Expiration Date: June 2007 S. Previdi Expiration Date: Jan 2008 S. Previdi
Cisco Systems Cisco Systems
Dec 2006 July 2007
IP Fast Reroute Using Not-via Addresses IP Fast Reroute Using Not-via Addresses
<draft-ietf-rtgwg-ipfrr-notvia-addresses-00.txt> <draft-ietf-rtgwg-ipfrr-notvia-addresses-01.txt>
Status of this Memo Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007). All rights reserved.
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.
Conventions used in this document 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].
Table of Contents Table of Contents
1. Overview of Not-via Repairs.........................................3 1. Overview of Not-via Repairs.........................................3
1.1 Use of Equal Cost Multi-Path......................................5 1.1 Use of Equal Cost Multi-Path.....................................5
1.2 Use of LFA repairs................................................5 1.2 Use of LFA repairs...............................................5
2. Not-via Repair Path Computation.....................................5 2. Not-via Repair Path Computation.....................................5
3. Operation of Repairs................................................6 3. Operation of Repairs................................................6
3.1 Node Failure......................................................6 3.1 Node Failure.....................................................7
3.2 Link Failure......................................................7 3.2 Link Failure.....................................................7
3.3 Multi-homed Prefix................................................7 3.3 Multi-homed Prefix...............................................8
3.4 Installation of Repair Paths......................................9 3.4 Installation of Repair Paths.....................................9
4. Compound failures..................................................10 4. Compound failures..................................................10
4.1 Shared Risk Link Groups..........................................10 4.1 Shared Risk Link Groups.........................................10
4.1.1 Use of LFAs with SRLGs.......................................14 4.1.1 Use of LFAs with SRLGs......................................14
4.2 Local Area Networks..............................................14 4.2 Local Area Networks.............................................14
4.2.1 Simple LAN Repair............................................15 4.2.1 Simple LAN Repair...........................................15
4.2.2 LAN Component Repair.........................................15 4.2.2 LAN Component Repair........................................16
4.2.3 LAN Repair Using Diagnostics.................................16 4.2.3 LAN Repair Using Diagnostics................................17
5. Multiple Simultaneous Failures.....................................17 5. Multiple Simultaneous Failures.....................................17
6. Optimizing not-via computations using LFAs.........................17 6. Optimizing not-via computations using LFAs.........................18
7. Multicast..........................................................18 7. Multicast..........................................................18
8. Fast Reroute in an MPLS LDP Network................................18 8. Fast Reroute in an MPLS LDP Network................................19
9. Encapsulation......................................................18 9. Encapsulation......................................................19
10. Routing Extensions................................................19 10. Routing Extensions................................................19
11. Incremental Deployment............................................19 11. Incremental Deployment............................................20
12. IANA considerations...............................................19 12. IANA considerations...............................................20
13. Security Considerations...........................................19 13. Security Considerations...........................................20
Introduction 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 [IPFRR], the routers that are the operating IP fast reroute [IPFRR], the routers that are the
neighbors of the failure repair the failure. These repairing routers neighbors of the failure repair the failure. These repairing routers
have to steer packets to their destinations despite the fact that have to steer packets to their destinations despite the fact that
most other routers in the network are unaware of the nature and most other routers in the network are unaware of the nature and
location of the failure. location of the failure.
A common limitation in most IPFRR mechanisms is an inability to A common limitation in most IPFRR mechanisms is an inability to
steer the repaired packet round an identified failure. The extent to indicate the identity of the failure and to explicitly steer the
which this limitation affects the repair coverage is topology repaired packet round the failure. The extent to which this
dependent. The mechanism proposed here is to encapsulate the packet limitation affects the repair coverage is topology dependent. The
to an address that explicitly identifies the network component that mechanism proposed here is to encapsulate the packet to an address
the repair must avoid. This produces a repair mechanism, which, that explicitly identifies the network component that the repair
provided the network is not partitioned by the failure, will always must avoid. This produces a repair mechanism, which, provided the
achieve a repair. network is not partitioned by the failure, will always achieve a
repair.
1. Overview of Not-via Repairs 1. Overview of Not-via Repairs
The purpose of a repair is to deliver packets to their destination The purpose of a repair is to deliver packets to their destination
without traversing a known failure in the network, i.e. to deliver without traversing a known failure in the network, i.e. to deliver
the packet not via the failure. A special address is assigned to the packet not via the failure. A special address is assigned to
each protected component. This address is called the not-via each protected component. This address is called the not-via
address. The semantics of a not-via address are that a packet address. The semantics of a not-via address are that a packet
addressed to a not-via address must be delivered to the router addressed to a not-via address must be delivered to the router
advertising that address, not via the protected component (link, advertising that address, not via (i.e. without traversing or
node, SRLG etc.) with which that address is associated. attempting to traverse) the protected component (link, node, SRLG
etc.) with which that address is associated.
A simple example would be node repair in which an additional address A simple example would be node repair in which an additional address
is assigned to each interface in the network. To repair a failure, is assigned to each interface in the network. To repair a failure,
the repairing router encapsulates the packet to the not-via address the repairing router encapsulates the packet to the not-via address
of the router interface on the far side of the failure. The routers of the router interface on the far side of the failure. The routers
on the repair path then know to which router they must deliver the on the repair path then know to which router they must deliver the
packet, and which network component they must avoid. The network packet, and which network component they must avoid. The network
fragment shown in Figure 1 illustrates a not-via repair for the case fragment shown in Figure 1 illustrates a not-via repair for the case
of a router failure. of a router failure.
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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
1.1 Use of Equal Cost Multi-Path 1.1 Use of Equal Cost Multi-Path
A router can use an equal cost multi-path (ECMP) repair in place of A router can use an equal cost multi-path (ECMP) repair in place of
a not-via repair. a 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.
1.2 Use of LFA repairs 1.2 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) as documented in [LFA]. (LFA) and or downstream paths as documented in [LFA].
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. The description in this that of the equivalent not-via repair for traffic destined to nodes
document assumes that LFAs will be used where available, but the close to the far end of the failure, but may be more optimal for
distribution of repairs between the two mechanisms is a local some other traffic. The description in this document assumes that
implementation choice. LFAs will be used where available, but the distribution of repairs
between the two mechanisms is a local implementation choice.
2. Not-via Repair Path Computation 2. 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.
3. Operation of Repairs 2.1 Computing not-via repairs in routing vector protocols
While this document focuses on link state routing protocols, it is
equally possible to compute not-via repairs in distance vector (e.g.
RIP) or path vector (e.g. BGP) routing protocols. This can be
achieved with very little protocol modification by advertising the
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
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 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
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 of achieving this is for P itself to perform the
suppression of any address of the form Xp.
3. 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.
3.1 Node Failure 3.1 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 recording the "next-next-hop" for each destination in addition to
the normal next-hop. the 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 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 of failure that partitioned the network, in which case ANY repair is
clearly impossible. clearly impossible.
3.2 Link Failure 3.2 Link Failure
The normal mode of operation of the network would be to assume The normal mode of operation of the network would be to assume
router failure. However, where some destinations are only reachable router failure. However, where some destinations are only reachable
through the failed router, it is desirable that an attempt be made through the failed router, it is desirable that an attempt be made
to repair to those destinations by assuming that only a link failure to repair to those destinations by assuming that only a link failure
has occurred. has occurred.
To perform a link repair, S encapsulates to Ps (i.e. it instructs 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 the 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 neighbors of S will have calculated a path to Ps in case S itself
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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. Referring to Figure 2, if A was
the neighbor of P that was on the link repair path from S to P, and the neighbor of P that was on the link repair path from S to P, and
P itself had failed, the repaired packet from S would arrive at A P itself had failed, the repaired packet from S would arrive at A
encapsulated to Ps. A would have detected that the AP link had encapsulated to Ps. A would have detected that the AP link had
failed and would normally attempt to repair the packet. However, no failed and would normally attempt to repair the packet. However, no
repair path is provided for any not-via address, and so A would be 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. forced to drop the packet, thus preventing the formation of loop.
3.3 Multi-homed Prefix 3.3 Multi-homed Prefix
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 [LFA]. Only those without such a repair using LFAs as described in [LFA]. Only those without such a repair
need be considered here. 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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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 may be specified by using a special address with the above
semantics. Note that only one such address is required per node. semantics. Note that only one such address is required per node.
Notice that using the not-via approach, only one level of Notice that using the not-via approach, only one level of
encapsulation was needed to repair MHPs to the alternate router. encapsulation was needed to repair MHPs to the alternate router.
3.4 Installation of Repair Paths 3.4 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. use in the event of a failure. It is assumed that the not-via repair
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 1. For all destinations with an ECMP or LFA repair (as described
in [LFA] ) install that repair. in [LFA] ) 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). will use to send the packet to DR). This can be determined
during the normal SPF run by recording the additional
3. For each next-next-hop node H for which S has a path to the information. If S has a path to the not-via address Hp (H not
not-via address Hp (H not via P), identify each destination via P), install a not-via repair to Hp for the destination DR.
with current next-next-hop H and install a not-via repair to Hp
for that destination.
4. 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 node is P, or a router for which P is a single point of
failure, and have an attachment point that is reachable after P failure, and have an alternative attachment point that is
has failed. reachable after P has failed. One way of determining these
destinations would be to run an SPF rooted at S with node P
removed, but an implementation may record alternative
attachment points during the normal SPF run. In either case,
the next best point of attachment can also be determined for
use in step (4) below.
5. For each multi-homed prefix (M) identified in step (4):- 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:-
i. Y, where the next hop towards Y is P, or i. Y, where the next hop towards Y is P, or
ii. Z, where the next hop towards Z is not P. ii. Z, where the next hop towards Z is not P.
b. If the attachment node is Z, install the repair for M as a b. 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).
c. For the subset of prefixes (M) that remain (having c. For the subset of prefixes (M) that remain (having
attachment point Y), install the repair path previously attachment point Y), install the repair path previously
installed for destination Y. installed for destination Y.
6. For each destination (DS) that remains, install a not-via 5. For each destination (DS) that remains, install a not-via
repair to Ps (P not via S). Note, these are destinations for repair to Ps (P not via S). Note, these are destinations for
which node P is a single point of failure, and can only be which node P is a single point of failure, and can only be
repaired by assuming that the apparent failure of node P was repaired by assuming that the apparent failure of node P was
simply a failure of the S-P link. simply a failure 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 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 is possible for a packet sent on the
downstream path to be returned to the sending node inside a
not-via encapsulation. Since packets destined to not-via
addresses are not repaired, the packet will be dropped after
executing a single turn loop.
4. Compound failures 4. Compound failures
4.1 Shared Risk Link Groups 4.1 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 In Figure 4 below, the links S-P and A-B are both members of
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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 complexity of performing the diagnostics, this requires
multiple not-via addresses per interface, which has poor multiple not-via addresses per interface, which has poor
scaling properties. scaling properties.
4.1.1 Use of LFAs with SRLGs 4.1.1 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 provided that any other link with which S-P shares an SRLG is
avoided when computing the LFA. This is described for the simple avoided when computing the LFA. This is described for the simple
case of "local-SRLGs" in [LFA]. case of "local-SRLGs" in [LFA].
4.2 Local Area Networks 4.2 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 between the sophistication of the fault detection and the size of
the SRLG. Protecting against link failure of the LAN link(s) is the SRLG. Protecting against link failure of the LAN link(s) is
relatively straightforward, but as with all fast reroute mechanisms, relatively straightforward, but as with all fast reroute mechanisms,
the problem becomes more complex when it is desired to protect the problem becomes more complex when it is desired to protect
against the possibility of failure of the nodes attached to the LAN against the possibility of failure of the nodes attached to the LAN
as well as the LAN itself. as well as the LAN itself.
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whether the failure is: whether the failure is:
. its own interface to the LAN, . its own interface to the LAN,
. the LAN itself, . the LAN itself,
. the LAN interface of P, . the LAN interface of P,
. the node P. . the node P.
4.2.1 Simple LAN Repair 4.2.1 Simple LAN Repair
A simple approach to LAN repair is to consider the LAN and all of 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 its 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 the LAN (Pl) would require P to be reached not-via any router
connected to the LAN. This is shown in Figure 11. connected to the LAN. This is shown in Figure 11.
Ql Cl Ql Cl
+-------------Q--------C +-------------Q--------C
| Qc | Qc
| |
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traffic reached via P and B to B not-via the LAN or any router traffic 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 attached to the LAN (i.e. to Bl). Any destination only reachable
through P would be addressed to P not-via the LAN or any router through P would be addressed to P not-via the LAN or any router
attached to the LAN (except of course P). attached to the 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.
4.2.2 LAN Component Repair 4.2.2 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 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. from S, i.e. to P not-via N. Similarly for routers Q and R.
skipping to change at page 16, line 40 skipping to change at page 17, line 22
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
4.2.3 LAN Repair Using Diagnostics 4.2.3 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.
5. Multiple Simultaneous Failures 5. Multiple Simultaneous Failures
The failure of a node or an SRLG can result in multiple correlated The failure of a node or an SRLG can result in multiple correlated
failures, which may be repaired using the mechanisms described in failures, which may be repaired using the mechanisms described in
this design. This design will not correctly repair a set of this design. This design will not correctly repair a set of
unanticipated multiple failures. Such failures are out of scope of unanticipated multiple failures. Such failures are out of scope of
this design and are for further study. this design and are for further study.
It is important that the routers in the network are able to It is important that the routers in the network are able to
discriminate between these two classes of failure, and take discriminate between these two classes of failure, and take
appropriate action. appropriate action.
6. Optimizing not-via computations using LFAs 6. Optimizing not-via computations using LFAs
If repairing node S has an LFA to the repair endpoint it is not If repairing node S has an LFA to the repair endpoint it is not
necessary for any router to perform the incremental SPF with the 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 link SP removed in order to compute the route to the not-via address
Ps. This is because the correct routes will already have been Ps. This is because the correct routes will already have been
computed as a result of the SPF on the base topology. Node S can computed as 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 signal this condition to all other routers by including a bit in its
LSP or LSA associated with each LFA protected link. Routers LSP or LSA associated with each LFA protected link. Routers
computing not-via routes can then omit the running of the iSPF for computing not-via routes can then omit the running of the iSPF for
links with this bit set. links with this bit set.
skipping to change at page 18, line 5 skipping to change at page 18, line 41
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.
7. 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 will be provided
in a future version of this draft. in a future version of this draft.
8. Fast Reroute in an MPLS LDP Network. 8. Fast Reroute in an MPLS LDP Network.
Not-via addresses are IP addresses and LDP [LDP] will distribute Not-via addresses are IP addresses and LDP [LDP] 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 first pushing the label which the repair endpoint uses to forward
the packet, and then pushing the label corresponding to the not-via the packet, and then pushing the label corresponding to the not-via
address needed to effect the repair. Referring once again to Figure 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 and B, 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 its S first pushes B's label for D. S then pushes the label that its
next hop to Bp needs to reach Bp. 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 [NNHL]. distribution mechanism proposed in [NNHL].
9. Encapsulation 9. Encapsulation
Any IETF specified IP in IP encapsulation may be used to carry a Any IETF specified IP in IP encapsulation may be used to carry a
not-via repair. IP in IP [IPIP], GRE [GRE] and L2TPv3 [L2TPv3], all not-via repair. IP in IP [IPIP], GRE [GRE] and L2TPv3 [L2TPv3], all
have the necessary and sufficient properties. The requirement is 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 encapsulated packet is addressed have a common ability to process
the chosen encapsulation type. the chosen encapsulation type.
When an MPLS LDP network is being protected, the encapsulation would When an MPLS LDP network is being protected, the encapsulation would
normally be an additional MPLS label. In an MPLS enabled IP network normally be an additional MPLS label. In an MPLS enabled IP network
an MPLS label may be used in place of an IP in IP encapsulation in an MPLS label may be used in place of an IP in IP encapsulation in
the case above. the case above.
10. 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 that the association between the protected component (link, router
or SRLG) and the not-via address can be determined by the other or SRLG) and the not-via address can be determined by the other
routers in the network. 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 [GRE], L2TPv3 etc). However, the that they support (MPLS, GRE [GRE], L2TPv3 etc). However, the
deployment of mixed IP encapsulation types within a network is deployment of mixed IP encapsulation types within a network is
discouraged. discouraged.
11. Incremental Deployment 11. Incremental Deployment
Incremental deployment is supported by excluding routers that are Incremental deployment is supported by excluding routers that are
not calculating not-via routes (as indicated by their capability not 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. capable.
Routers that are protecting a network component need to have the Routers that are protecting a network component need to have the
capability to encapsulate and de-capsulate packets. However, routers capability to encapsulate and de-capsulate packets. However, routers
that are on the repair path only need to be capable of calculating that are on the repair path only need to be capable of calculating
not-via paths and including the not-via addresses in their FIB i.e. not-via paths and including the not-via addresses in their FIB i.e.
these routers do not need any changes to their forwarding mechanism. these routers do not need any changes to their forwarding mechanism.
12. 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.
13. Security Considerations 13. Security Considerations
The repair endpoints present vulnerability in that they might be The repair endpoints present vulnerability in that they might be
used as a method of disguising the delivery of a packet to a point used as a method of disguising the delivery of a packet to a point
in the network. The primary method of protection should be through in the network. The primary method of protection should be through
the use of a private address space for the not-via addresses. These the use 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 [GRE] [L2TPv3]. With the deployment of such through the use of a key [GRE] [L2TPv3]. With the deployment of such
mechanisms, the repair endpoints would not increase the security mechanisms, the repair endpoints would not increase the security
skipping to change at page 21, line 5 skipping to change at page 21, line 26
of such proprietary rights by implementers or users of this of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr. at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf- this standard. Please address the information to the IETF at ietf-
ipr@ietf.org. ipr@ietf.org.
Full copyright statement Disclaimer of Validity
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE. FOR A PARTICULAR PURPOSE.
Copyright statement
Copyright (C) The IETF Trust (2007). This document is subject to the
rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.
Normative References Normative References
There are no normative references. There are no normative references.
Informative References Informative References
Internet-drafts are works in progress available from Internet-drafts are works in progress available from
<http://www.ietf.org/internet-drafts/> <http://www.ietf.org/internet-drafts/>
[BFD] Katz, D., Ward, D., "Bidirectional Forwarding [BFD] Katz, D., Ward, D., "Bidirectional Forwarding
Detection", < draft-ietf-bfd-base-05.txt>, Detection", < draft-ietf-bfd-base-06.txt>,
June 2006, (work in progress). March 2007, (work in progress).
[GRE] S. Hanks, T. Li, D. Farinacci, P. Traina. [GRE] S. Hanks, T. Li, D. Farinacci, P. Traina.
"Generic Routing Encapsulation (GRE)", "Generic Routing Encapsulation (GRE)",
RFC 1701,. October 1994. RFC 1701,. October 1994.
[IPFRR] Shand, M., Bryant, S., "IP Fast-reroute [IPFRR] Shand, M., Bryant, S., "IP Fast-reroute
Framework", Framework",
<draft-ietf-rtgwg-ipfrr-framework-06.txt>, <draft-ietf-rtgwg-ipfrr-framework-07.txt>,
October 2006, (work in progress). June 2007, (work in progress).
[IPIP] Perkins, C., "IP encapsulation within IP", RFC [IPIP] Perkins, C., "IP encapsulation within IP", RFC
2003, October 1996 2003, October 1996
[ISPF] McQuillan, J., I. Richer and E. Rosen, "ARPANET [ISPF] McQuillan, J., I. Richer and E. Rosen, "ARPANET
Routing Algorithm Improvements", BBN Technical Routing Algorithm Improvements", BBN Technical
Report 3803, April 1978. Report 3803, April 1978.
[L2TPv3] J. Lau, Ed., et al., "Layer Two Tunneling [L2TPv3] J. Lau, Ed., et al., "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, Protocol - Version 3 (L2TPv3)", RFC 3931,
March 2005. March 2005.
[LDP] Andersson, L., Doolan, P., Feldman, N., [LDP] Andersson, L., Doolan, P., Feldman, N.,
Fredette, A. and B. Thomas, Fredette, A. and B. Thomas,
"LDP Specification", RFC 3036, January 2001. "LDP Specification", RFC 3036, January 2001.
[LFA] A. Atlas, Ed, A. Zinin, Ed, "Basic [LFA] A. Atlas, Ed, A. Zinin, Ed, "Basic
Specification for IP Fast-Reroute: Loop-free Specification for IP Fast-Reroute: Loop-free
Alternates", Alternates",
<draft-ietf-rtgwg-ipfrr-spec-base-05.txt>, <draft-ietf-rtgwg-ipfrr-spec-base-06.txt>,
Feb 2006, (work in progress). March 2007, (work in progress).
[MPLS-TE] Ping Pan, et al, "Fast Reroute Extensions to
RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[NNHL] Shen, N., et al "Discovering LDP Next-Nexthop [NNHL] Shen, N., et al "Discovering LDP Next-Nexthop
Labels", Labels",
<draft-shen-mpls-ldp-nnhop-label-02.txt>, <draft-shen-mpls-ldp-nnhop-label-02.txt>,
May 2005, (work in progress) May 2005, (work in progress)
[RFC2119] Bradner, S., "Key words for use in RFCs to [RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", RFC 2119 Indicate Requirement Levels", RFC 2119
(BCP 14), March 1997 (BCP 14), March 1997
 End of changes. 56 change blocks. 
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