One document matched: draft-kompella-ospf-multiaccess-te-00.txt
Network Working Group K. Kompella
Internet Draft Juniper Networks
Updates: 3630 October 2003
Category: Standards Track Expires: April 2004
OSPF v2 Traffic Engineering Extensions for Multi-access Networks
draft-kompella-ospf-multiaccess-te-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo specifies Traffic Engineering extensions for OSPF version 2
for dealing with multi-access networks. In particular, the bandwidth
attributes in the original OSPFv2 TE specification do not accurately
model the available bandwidth across a multi-access network; this
memo rectifies this shortcoming.
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Internet Draft OSPF TE for Multi-access Networks October 2003
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
1. Introduction
RFC 3630 [2] specifies how Traffic Engineering (TE) properties of
links can be carried in OSPF v2 Opaque Link State Advertisements
(LSAs) [3], but explicits states (in section 1.2) that "The
reservation state of multi-access links may not be accurately
reflected". This is the case for at least two reasons:
(a) the Designated Router (DR) doesn't generate an Opaque LSA to
define the reservation state of links _towards_ routers in a
multi-access network;
(b) even if it did, it would be hard for the DR to keep track of
this reservation state.
Consider the following multi-access network N:
A B C D
| | | |
| | | |
N ---------------------------------------------
| | |
| | |
E F G
This is modelled in OSPF (and OSPF-TE) as:
B C
\ /
\ /
\ /
A ----- X ----- D
/ | \
/ | \
/ | \
E F G
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where one of the routers A-G is elected the DR, here called X.
In OSPF-TE, each of the routers A through G advertises its available
(unreserved) bandwidth towards X by means of a TE LSA. X, on the
other hand, does not advertise a TE LSA, instead advertising a
regular network LSA per the OSPF v2 protocol [4], section 7.3.
Routers computing paths across network N simply assume that there is
always sufficient bandwidth from X to each of the routers A through
G.
Consider the case where N is implemented by a switch such that each
router has a full-duplex 100Mbps connection to the switch, and
advertises 100Mbps in its TE LSAs. A remote router R computing a
100Mbps path would see that 100Mbps is available from A to X, and
ignoring the X->D link, could set up a path via A and D.
Subsequently, another remote router Q looking at the B->X->D links
would conclude that there was sufficient bandwidth for a 100Mbps path
via B->D. However, this would put 200Mbps of traffic on the 100Mbps
link from the switch to D.
This memo attempts to rectify this shortcoming of [2].
2. Approach
A simple approach would be for the DR to advertise TE LSAs for links
from itself to other routers. This is undesirable for at least three
reasons:
(a) This change is not backward compatible with [2].
(b) The only relevant TE information on the links advertised from
the DR to other routers is the available bandwidth.
Advertising a metric or administrative groups from X to A, for
example, would change the topology of the overall network.
(c) It is non-trivial for the DR to track the available bandwidth
on other routers' links. In the above example, how would the
DR know how much traffic has been reserved on the link from N
to D?
The approach taken here is for each router (say A) to advertise in
its TE LSA for its link to the DR (i.e., A->X) the available "reverse
bandwidth" (i.e., X to A).
Going back to the example above, each router A-G would advertise its
available forward bandwidth (100Mbps), as well as its available
reverse bandwidth (100Mbps). When router R set up the path via
A->X->D, D would advertise that its available reverse bandwith (D<-X)
was now zero. Hence router Q would not attempt to use the X->D link.
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3. Encoding in OSPF v2 TE LSAs
The Reverse Bandwidth sub-TLV of the TE Link TLV is encoded in OSPF
v2 as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD by IANA) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Media Type | Reserved (sbz) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Available Bandwidth at Index 0 |
. .
. .
. .
| Reverse Available Bandwidth at Index k |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"sbz" means SHOULD be zero when sending, SHOULD be ignored on
receipt.
The Type is [TBD]. The Length is equal to 4 + 4*(k+1) octets.
3.1. Types of Multiaccess Media
Media Type values are given in the following table:
Value Media Type
----- ----------
0 Unknown
1 Shared
2 Switched Half Duplex
3 Switched Full Duplex
4-255 Reserved
An example of a "Shared" link is a CSMA/CD Ethernet connection ([5]).
An example of a "Switched Half Duplex" link is a half duplex port on
a switched Ethernet ([6]). An example of a "Switched Full Duplex"
link is a full duplex port on a switched Ethernet.
3.2. Reverse Available Bandwidth
The semantics of the "Reverse Available Bandwidth at Index i"
parallels exactly the available bandwidth at index i as advertised in
the "Unreserved Bandwidth" field, except that it is in the reverse
direction (DR to router, rather than router to DR). The format and
units are identical to that of the "Unreserved Bandwidth" field.
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3.3. Consistency Checks
A Reverse Bandwidth sub-TLV within a TE Link TLV is ignored in its
entirety if the link's Link Type is not 'Multiaccess' (value 2).
Consider the set of links belonging to a given multi-access network.
Not all the corresponding TE Link TLVs need have a Reverse Bandwidth
sub-TLV. However those that do MUST have compatible Media Types,
i.e., they MUST all be "Shared"; or they MUST all be either "Switched
Half Duplex" or "Switched Full Duplex". If this test fails, all
Reverse Bandwidth sub-TLVs for the set of links connected to this DR
SHOULD be ignored.
If a new Media Type is defined, so must corresponding consistency
checks for the new type.
For multi-access links with Media Type "Shared" or "Switched Half
Duplex", the Reverse Available Bandwidths SHOULD NOT be advertised,
and SHOULD be ignored on receipt (i.e., k = -1, and the Reverse
Bandwidth TLV Length SHOULD be 4).
For multi-access links with Media Type "Switched Full Duplex", if the
Unreserved Bandwidth sub-TLV (type 8) of the Link TLV contains
priority levels 0 through n, then k SHOULD be equal to n. Values of
Reverse Available Bandwidth at index i > n (if any) SHOULD be
ignored. If k < n, then computation of available bandwidth for index
i, k < i < n, will perforce be inaccurate.
4. Computing Available Bandwidth
This section illustrates how the information in the Reverse Bandwidth
TLV can be used to compute the available bandwidth in the direction
DR->router. It is however not meant as a primer on Constraint-based
Routing.
4.1. Shared Media
For each link l in multi-access network N of type Shared, let Ml be
the Maximum Reservable Bandwidth (sub-TLV 7 of the Link TLV) for l,
and let Ul be the Unreserved Bandwidth at index i for l.
Let M be min (Ml) over all links l in N. The available bandwidth at
index i Ai across N (independent of which pair of routers in N) is
given by
Ai = M - sum (Ml - Ul) (sum taken over all links l in N)
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4.2. Switched Media
Consider two routers A and B and a designated router X in a multi-
access network N. Call the link from A to X 'a' and the link from B
to X 'b'. The available bandwidth at index i from A to B when the
Media Type of b is "Switched Half Duplex" is given by;
min (Ua, Ub)
where Ua is the Unreserved Bandwidth at index i for link a, and Ub is
the Unreserved Bandwidth at index i for link b.
If link b is of Media Type "Switched Full Duplex", the available
bandwidth at index i from A to B is given by:
min (Ua, Rb)
where Ua is the Unreserved Bandwidth at index i for link a, and Rb is
the Reverse Available Bandwidth at index i for link b.
If link b doesn't have a Reverse Bandwidth sub-TLV, it is treated as
a link of type "Switched Half Duplex".
Note 1: if a flow of bandwidth B at index i in any direction is
successfully admitted on a link of Media Type "Switched Half Duplex",
then the Unreserved Bandwidth at index i on that link is decremented
by B.
Note 2: if a flow of bandwidth B at index i in the direction towards
the DR is successfully admitted on a link of Media Type "Switched
Full Duplex", then the Unreserved Bandwidth at index i on that link
is decremented by B. If the flow is away from the DR, the Reverse
Available Bandwidth at index i on that link is decremented by B.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[2] Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering
Extensions to OSPF Version 2", RFC 3630, September 2003
[3] Coltun, R., "The OSPF Opaque LSA Option," RFC 2370, July 1998.
[4] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
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Informative References
[5] Metcalfe, R. M. and D. R. Boggs, "Ethernet: Distributed Packet
Switching for Local Computer Networks", Communications of the
ACM, Vol. 19, No. 5, July 1976, pp. 395 - 404
[6] Spurgeon, C. E., "Ethernet: The Definitive Guide", O'Reilly &
Associates, February 2000
Security Considerations
No new security vulnerabilities are introduced by this document.
IANA Considerations
IANA must assign a new sub-TLV of the Link TLV for the Reverse
Bandwidth sub-TLV from the space allocated for Standards Action.
Authors' Addresses
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
EMail: kireeti@juniper.net
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Internet Draft OSPF TE for Multi-access Networks October 2003
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