Network Working Group Naiming Shen
INTERNET DRAFT Redback Networks
Category: Informational Henk Smit
Expiration Date: November 2004
May 2004
Calculating IGP Routes Over Traffic Engineering Tunnels
draft-ietf-rtgwg-igp-shortcut-01.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This document describes how conventional hop-by-hop link-state
routing protocols interact with new Traffic Engineering capabilities
to create IGP shortcuts. In particular this document describes how
Dijkstra's SPF algorithm can be adapted so that link-state IGPs
will calculate IP routes to forward traffic over tunnels that are
set up by Traffic Engineering.
3. Introduction
Link-state protocols like integrated IS-IS [1] and OSPF [2] use
Dijkstra's SPF algorithm to compute a shortest path tree to all nodes
in the network. Routing tables are derived from this shortest path
tree. The routing tables contain tuples of destination and first-hop
information. If a router does normal hop-by-hop routing, the first-
hop will be a physical interface attached to the router.
New traffic engineering algorithms calculate explicit routes to one
Shen, Smit Expires November 2004 [Page 1]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
or more nodes in the network. At the router that originates explicit
routes, such routes can be viewed as logical interfaces which supply
Label Switched Paths through the network. In the context of this
document we refer to these Label Switched Paths as Traffic
Engineering tunnels (TE-tunnels). Such capabilities are specified
in [3] and [4].
The existence of TE-tunnels in the network and how the traffic
in the network is switched over those tunnels are orthogonal
issues. A node may define static routes pointing to the TE-tunnels;
it may match the recursive route next-hop with the TE-tunnel
end-point address; or it may define local policy such as affinity
based tunnel selection for switching certain traffic. This document
describes a mechanism utilizing link-state IGPs to dynamically
install IGP routes over those TE-tunnels.
The tunnels under consideration are tunnels created explicitly by
the node performing the calculation, and with an end-point address
known to this node. For use in algorithms such as the one described
in this document, it does not matter whether the tunnel itself is
strictly or loosely routed. A simple constraint can ensure that the
mechanism being loop free. When a router chooses to inject a packet
addressed to a destination D, the router may inject the packet into
a tunnel where the end-point is closer, according to link-state
IGP topology, to the destination D than the injecting router is.
In other words, the tail-end of the tunnel has to be a downstream
IGP node for the destination D. The algorithms that follow are one
way that a router may obey this rule and dynamically make
intelligent choices about when to use TE-tunnels for traffic.
This algorithm may be used in conjunction with other mechanisms
such as statically defined routes over TE-tunnels or traffic flow
and QoS based TE-tunnel selection.
This IGP shortcut mechanism assumes the TE-tunnels have already
been setup. The TE-tunnels in the network may be used for
QoS, bandwidth, redundancy or fastreroute reasons. When IGP
shortcut mechanism is applied on those tunnels, or other
mechanisms are used in conjunction with IGP shortcut, the
physical traffic switching through those tunnels may not
match the initial traffic engineering setup goal. Also the
traffic pattern in network may change with time. Some forwarding
plane measurement and feedback into the adjustment of TE-tunnel
attributes need to be there to ensure the network being
traffic engineered efficiently [6].
4. Enhancement to the Shortest Path First computation
During each step of the SPF computation, a router discovers the path
to one node in the network. If that node is directly connected to the
calculating router, the first-hop information is derived from the
Shen, Smit Expires November 2004 [Page 2]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
adjacency database. If a node is not directly connected to the
calculating router, it inherits the first-hop information from the
parent(s) of that node. Each node has one or more parents. Each node
is the parent of zero or more down-stream nodes.
For traffic engineering purposes each router maintains a list of all
TE-tunnels that originate at this router. For each of those TE-
tunnel, the router at the tail-end is known.
During SPF, when a router finds the path to a new node (in other
words, this new node is moved from the TENTative list to the PATHS
list), the router must determine the first-hop information. There
are three possible ways to do this:
- Examine the list of tail-end routers directly reachable via a
TE-tunnel. If there is a TE-tunnel to this node, we use the
TE-tunnel as the first-hop.
- If there is no TE-tunnel, and the node is directly connected, we
will use the first-hop information from the adjacency database.
- If the node is not directly connected, and is not directly
reachable via a TE-tunnel, we will copy the first-hop
information from the parent node(s) to the new node.
The result of this algorithm is that traffic to nodes that are the
tail-end of TE-tunnels, will flow over those TE-tunnels. Traffic to
nodes that are downstream of the tail-end nodes will also flow over
those TE-tunnels. If there are multiple TE-tunnels to different
intermediate nodes on the path to destination node X, traffic will
flow over the TE-tunnel whose tail-end node is closest to node X.
In certain applications, there is a need to carry both the native
adjacency and the TE-tunnel next-hop information for the TE-tunnel
tail-end and its downstream nodes. The head-end node may
conditionally switch the data traffic onto TE-tunnels based on
user defined criteria or events; The head-end node may also split
flow of traffic towards either types of the next-hops; The head-end
node may install the routes with two different types of next-hops
into two separate RIBs. Multicast protocols running over physical
links may have to perform RPF checks using the native adjacency
next-hops rather than the TE-tunnel next-hops.
5. Special cases and exceptions
The Shortest Path First algorithm will find equal-cost parallel paths
to destinations. The enhancement described in this document does not
change this. Traffic can be forwarded over one or more native IP
paths, over one or more TE-tunnels, or over a combination of native
IP paths and TE-tunnels.
Shen, Smit Expires November 2004 [Page 3]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
A special situation occurs in the following topology:
rtrA -- rtrB -- rtrC
| |
rtrD -- rtrE
Assume all links have the same cost. Assume a TE-tunnel is set up
from rtrA to rtrD. When the SPF calculation puts rtrC on the
TENTative list, it will realize that rtrC is not directly connected,
and thus it will use the first-hop information from the parent. Which
is rtrB. When the SPF calculation on rtrA moves rtrD from the
TENTative list to the PATHS list, it realizes that rtrD is the
tail-end of a TE-tunnel. Thus rtrA will install a route to rtrD via
the TE-tunnel, and not via rtrB.
When rtrA puts rtrE on the TENTative list, it realizes that rtrE is
not directly connected, and that rtrE is not the tail-end of a TE-
tunnel. Therefor rtrA will copy the first-hop information from the
parents (rtrC and rtrD) to the first-hop information of rtrE.
Traffic to rtrE will now load-balance over the native IP path via
rtrA->rtrB->rtrC, and the TE-tunnel rtrA->rtrD.
In the case where both parallel native IP paths and paths over TE-
tunnels are available, implementations can allow the network
administrator to force traffic to flow over only TE-tunnels (or only
over native IP paths) or both to be used for load sharing.
6. Metric adjustment of IP routes over TE-tunnels
When an IGP route is installed in the routing table with a TE-tunnel
as next hop, an interesting question is what should be the cost or
metric of this route ? The most obvious answer is to assign a metric
that is the same as the IGP metric of the native IP path as if the
TE-tunnels did not exist. For example, rtrA can reach rtrC over a
path with a cost of 20. X is an IP prefix advertised by rtrC. We
install the route to X in rtrA's routing table with a cost of 20.
When a TE-tunnel from rtrA to rtrC comes up, by default the route is
still installed with metric of 20, only the next-hop information for
X is changed.
While this scheme works well, in some networks it might be useful to
change the cost of the path over a TE-tunnel, to make the route over
the TE-tunnel less or more preferred than other routes.
For instance when equal cost paths exist over a TE-tunnel and over a
native IP path, by adjusting the cost of the path over the TE-tunnel,
we can force traffic to prefer the path via the TE-tunnel, to prefer
the native IP path, or to load-balance among them. Another example is
when multiple TE-tunnels go to the same or different destinations.
Adjusting TE-tunnel metrics can force the traffic to prefer some
Shen, Smit Expires November 2004 [Page 4]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
TE-tunnels over others regardless of underlining IGP cost to those
destinations.
Setting a manual metric on a TE-tunnel does not impact the SPF
algorithm itself. It only affects comparison of the new route with
existing routes in the routing table. Existing routes can be either
IP routes to another router that advertises the same IP prefix, or it
can be a path to the same router, but via a different outgoing
interface or different TE-tunnel. All routes to IP prefixes
advertised by the tail-end router will be affected by the TE-tunnel
metric. Also the metrics of paths to routers that are downstream of
the tail-end router will be influenced by the manual TE-tunnel
metric.
This mechanism is loop free since the TE-tunnels are source-routed
and the tunnel egress is a downstream node to reach the computed
destinations. The end result of TE-tunnel metric adjustment is
more control over traffic loadsharing. If there is only one way
to reach a particular IP prefix through a single TE-tunnel, then no
matter what metric is assigned, the traffic has only one path to go.
The routing table described in this section can be viewed as the
private RIB for the IGP. The metric is an important attribute to
the routes in the routing table. A path or paths with lower metric
will be selected over other paths for the same route in the
routing table.
6.1. Absolute and relative metrics
It is possible to represent the TE-tunnel metric in two different
ways: an absolute (or fixed) metric or a relative metric, which is
merely an adjustment of the dynamic IGP metric as calculate by the
SPF computation. When using an absolute metric on a TE-tunnel, the
cost of the IP routes in the routing table does not depend on the
topology of the network. Note that this fixed metric is not only used
to compute the cost of IP routes advertised by the router that is the
tail-end of the TE-tunnel, but also for all the routes that are
downstream of this tail-end router. For example, if we have TE-
tunnels to two core routers in a remote POP, and one of them is
assigned with absolute metric of 1, then all the traffic going to
that POP will traverse this low-metric TE-tunnel.
By setting a relative metric, the cost of IP routes in the routing
table is based on the IGP metric as calculated by the SPF
computation. This relative metric can be a positive or a negative
number. Not configuring a metric on a TE-tunnel is a special case of
the relative metric scheme. No metric is the same as a relative
metric of 0. The relative metric is bounded by minimum and maximum
allowed metric values while the positive metric disables the
TE-tunnel in the SPF calculation.
Shen, Smit Expires November 2004 [Page 5]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
6.2. Examples of metric adjustment
Assume the following topology. X, Y and Z are IP prefixes advertised
by rtrC, rtrD and rtrE respectively. T1 is a TE-tunnel from rtrA to
rtrC. Each link in the network has an IGP metric of 10.
===== T1 =====>
rtrA -- rtrB -- rtrC -- rtrD -- rtrE
10 10 | 10 | 10 |
X Y Z
Without TE-tunnel T1, rtrA will install IP routes X, Y and Z in the
routing table with metrics 20, 30 and 40 respectively. When rtrA has
brought up TE-tunnel T1 to rtrC, and if rtrA is configured with the
relative metric of -5 on tunnel T1, then the routes X, Y, and Z will
be installed in the routing table with metrics 15, 25, and 35. If an
absolute metric of 5 is configured on tunnel T1, then rtrA will
install routes X, Y and Z all with metrics 5, 15 and 25 respectively.
7. Security Considerations
This document does not change the security aspects of IS-IS or OSPF.
Security considerations specific to each protocol still apply. For
more information see [5] and [2].
8. Acknowledgments
The authors would like to thank Joel Halpern and Christian Hopps for
their comments to this document.
9. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
Shen, Smit Expires November 2004 [Page 6]
Internet Draft IGP ShortCut Over MPLS LSPs May 2004
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
10. References
[1] ISO. Information Technology - Telecommunications and
Information Exchange between Systems - Intermediate System
to Intermediate System Routing Exchange Protocol for
Use in Conjunction with the Protocol for Providing the
Connectionless-Mode Network Service. ISO, 1990.
[2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[3] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J. McManus,
"Requirements for Traffic Engineering Over MPLS", RFC 2702,
September 1999.
[4] D. Awduche, et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC 3209, December 2001.
[5] T. Li, R. Atkinson, "Intermediate System to Intermediate System
(IS-IS) Cryptographic Authentication", RFC 3567, July 2003.
[6] D. Awduche, A. Chiu, A. Elwalid, I. Widjaja, X. Xiao,
"Overview and Principles of Internet Traffic Engineering,"
RFC-3272, May 2002.
11. Authors' Addresses
Naiming Shen
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
Email: naiming@redback.com
Henk Smit
Email: hhwsmit@xs4all.nl
Shen, Smit Expires November 2004 [Page 7]
