Internet DRAFT - draft-kvk-trill-fair-share-af-load-share
draft-kvk-trill-fair-share-af-load-share
INTERNET-DRAFT Kesava Vijaya Krupakaran
Intended Status: Proposed Standard Janardhanan Pathangi Narasimhan
Expires: March 25, 2013 Dell
September 21, 2012
Fair Share AF Load Share
draft-kvk-trill-fair-share-af-load-share-02
Abstract
In an access LAN of a TRILL campus, the DRB can choose to load share
the AF responsibility among other RBridges in the LAN. This document
throws light on one such approach where the AF appointment is fair
share scheduled among the RBridges.
Status of this Memo
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Shares . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 AF Affinity VLAN Set . . . . . . . . . . . . . . . . . . . . . 4
4 AF Affinity VLAN Set Overlap . . . . . . . . . . . . . . . . . 5
5 AF Distribution Among Heterogeneous RBridges . . . . . . . . . 6
6 AF Computation at DRB . . . . . . . . . . . . . . . . . . . . . 6
7 AF and VLAN Mapping . . . . . . . . . . . . . . . . . . . . . . 7
8 AF and Multiple ports on a link . . . . . . . . . . . . . . . . 7
9 Multi-Topology-Aware Port Capability Sub-TLVs . . . . . . . . . 7
9.1 Fair Share Sub-TLV . . . . . . . . . . . . . . . . . . . . 7
9.2 AF Affinity VLAN Set Sub-TLV . . . . . . . . . . . . . . . 7
9.3 Partial VLANs Appointing Sub-TLV . . . . . . . . . . . . . 8
10 Security Considerations . . . . . . . . . . . . . . . . . . . 9
11 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12 References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1 Normative References . . . . . . . . . . . . . . . . . . . 9
12.2 Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1 Introduction
In a shared access LAN, the appointed forwarder for a VLAN is
responsible for encapsulating and decapsulating native traffic on
that VLAN. Other non-AF RBridges in the LAN discard the native
traffic for that VLAN.
The DRB can choose to be the AF for all VLANs or load share the AF
responsibility among other RBRidges in the LAN. This ensures better
utilization of resources like hardware tables and buffers. The VLAN
partitioning scheme suggested in [RFC6439] section 2.2.1 is static
and requires careful configuration. Another simple protocol would be
to allocate VLANs in a round-robin fashion among all RBridges in the
LAN. However this doesn't leave scope for schemes like retaining 50%
of VLANs with the DRB and distribute only the rest among others.
Fair share scheduling of AF allows for the flexibility of assigning
certain RBridges (say with higher switching capability) AF for higher
proportion of VLANs than others.
1.1 Terminology
This document uses the acronyms defined in [RFC6439].
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 [RFC2119].
2 Shares
Each RBridge is configured with certain quantity of shares. A share
is the proportion of VLANs which would be allocated to the RBridge in
comparison with other RBridges. The face value of the shares is a
relative quantity and makes sense only when taken in conjunction with
total shares allocated in the LAN.
These shares are advertised by each RBridge in its hello. The DRB
load shares the AF among RBridges based on the relative value of
shares.
For instance, let A, B and C be three RBridges with S(A) = 2, S(B) =
1 and S(C) = 1. Then A is assigned the AF for 1/2 of the VLANs while
B and C the AF for 1/4th of the VLANs each.
Even when the number of VLANs for which the RBridge is to be AF
calculates to a non integer value, it should be made sure that there
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is only one AF for a VLAN in a multi-access LAN.
3 AF Affinity VLAN Set
Fair share scheduling distributes VLANs among RBridges according to
proportion of shares allocated. This allows allocation of higher
proportion of VLANs to certain RBridges (with higher switching
capability). However, this does not guarantee that these RBridges
would handle larger share of the native traffic.
Following the previous example, even though A is appointed AF for 50%
of the VLANs while B only 25% of the VLANs, the traffic load of VLANs
for which B is AF could be considerably higher that those in A.
In order to overcome this conundrum, each RBriged in access LAN is
configured with an AF Affinity VLAN Set apart form the share
proportion. This RBridge has AF affinity to the set of configured
VLANs. Thus when the DRB appoints an RBridge AF for a set of VLANs,
the members of the set are chosen from the AF Affinity VLAN Set
advertised.
Expanding on the previous example, if X denotes an RBridge, let S(X)
be the shares assigned to X, V(X) be the AF Affinity VLAN Set and
AF(X) denote the set of VLANs for which X is assigned AF. Let the
access LAN encompass ten shared VLANs [11, 20]. In this case the AF
assignment with just the shares configured could be as in Table 1. If
RBridge A has higher switching capability and VLANs [16, 20] are
heavily loaded, this AF appointment defeats the purpose.
+----------------------------------------------------+
|Table 1: AF appointment using fair share scheduling |
+--------+-------------+-----------------------------+
| X | S(X) | AF(X) |
+--------+-------------+-----------------------------+
| A | 2 | {11, 12, 13, 14, 15} |
+--------+-------------+-----------------------------+
| B | 1 | {16, 17, 18} |
+--------+-------------+-----------------------------+
| C | 1 | {19, 20} |
+--------+-------------+-----------------------------+
By configuring AF Affinity VLAN set in each RBridge, this difficulty
can be overcome. Such a configuration is shown in Table 2. How the AF
Affinity VLAN set is arrived at is beyond the scope of this document.
Long term traffic planning tools could be helpful in extrapolating a
decent configuration.
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+----------------------------------------------------------+
|Table 2: Fair share scheduling with AF Affinity VLAN set |
+--+-------+-----------------------+-----------------------+
|X | S(X) | V(X) | AF(X) |
+--+-------+-----------------------+-----------------------+
|A | 2 | {16, 17, 18, 19, 20} | {16, 17, 18, 19, 20} |
+--+-------+-----------------------+-----------------------+
|B | 1 | {11, 12, 13, 14} | {11, 12, 13} |
+--+-------+-----------------------+-----------------------+
|C | 1 | {12, 13, 14} | {14, 15} |
+--+-------+-----------------------+-----------------------+
4 AF Affinity VLAN Set Overlap
If the AF Affinity VLAN sets advertised by the RBridges overlap, the
RBridge with higher share has priority over the affinity of common
VLANs. In case the RBridges advertise same share with conflicting AF
Affinity VLAN sets, then the one with higher system ID gets more AF
affinity over the common VLANs.
+-------------------------------------------------------------------+
| Table 3: Fair share scheduling with AF Affinity VLAN set overlap |
+-+--------------+----+------------------------+--------------------+
|X| ID(X) |S(X)| V(X) | AF(X) |
+-+--------------+----+------------------------+--------------------+
|A|0000.0000.000a| 2 |{15, 16, 17, 18, 19, 20}|{15, 16, 17, 18, 19}|
+-+--------------+----+------------------------+--------------------+
|B|0000.0000.000b| 1 | {11, 12, 13, 14, 15} | {14, 20} |
+-+--------------+----+------------------------+--------------------+
|C|0000.0000.000c| 1 | {11, 12, 13, 14} | {11, 12, 13} |
+-+--------------+----+------------------------+--------------------+
Consider the previous examples with the LAN comprising of three
RBridges A, B and C coloured for VLANs [11, 20]. As shown in table 3,
the AF Affinity VLAN sets overlap in RBridges {A, C} as well as {B,
C}. A, having the highest share has the most affinity over the VLANs
configured there. In this example, A has higher AF affinity to VLAN
15 than C. Similarly, C has greater the AF affinity of VLANs [11, 14]
than B on virtue of its higher system ID.
It is possible to calculate a better AF distribution by examining
common VLANs in AF Affinity VLAN sets when they overlap. Such
algorithms have been avoided to keep the computation at DRB simple.
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5 AF Distribution Among Heterogeneous RBridges
An access LAN could constitute motley set of RBridges with some that
support fair share AF scheduling and some that doesn't. If the DRB
doesn't support fair share AF scheduling, it ignores the sub-TLVs
advertised by other RBridges and continue to distribute AF as it did
previously.
If the DRB does support fair share AF scheduling and it receives
hello form an RBridge without Fair Share Sub-TLV, it is assigned a
default share equal to average of all shares advertised in the LAN
during AF computation. If the AF Affinity VLAN Set Sub-TLV was not
advertised, it is taken to be a NULL set. In case an RBridge
advertises AF Affinity Sub-TLV without saying the shares, such TLV is
ignored and the behaviour follows as though it had not advertised AF
Affinity VLAN set.
In particular, if DRB is the only RBridge supporting the feature, all
the RBridges get equal shares (equal to the one configured at DRB,
consistent with the average rule discussed).
For instance, in an access LAN with RBridges A, B and C where S(A) =
7, S(B) = 3, and C doesn't support fair share AF scheduling, the DRB
assigns it a default of 5 shares.
As a special case, if DRB supports fair share AF load share and none
of the RBridges advertise any share and no share is configured in
DRB, then DRB assigns a share value of 1 to all RBridges and load
shares VLANs equally among all the RBridges.
6 AF Computation at DRB
DRB runs through all RBridges, in descending order of shares
configured and assigns the AF based on Affinity VLAN set. If the
shares advertised are equal, then RBridges are ordered based on
system ID. If RBridges don't advertise shares, they are assigned
default shares and are placed below RBridges who advertise shares in
the ordered list of RBridges. If there are multiple such non
congruous RBridges, they are again ordered based on system ID.
The DRB also monitors hellos for any change from previously
advertised shares or AF Affinity VLAN set. If it detects a change,
the AF assignment is recomputed for all RBridges. Any addition or
deletion of adjacency also triggers fresh AF assignment. This
simplifies the computation at DRB.
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7 AF and VLAN Mapping
If the DRB detects VLAN mapping, it appoints one RBridge (possibly
itself) as the AF for all VLANs as suggested in [RFC6439] section 2.4
to prevent loops.
8 AF and Multiple ports on a link
The shares configured represents the whole RBridge's proportion of AF
sought. Further load sharing of AF among multiple ports on same link
in an RBridge is a local decision.
9 Multi-Topology-Aware Port Capability Sub-TLVs
Two new Multi-Topology-Aware Port Capability Sub-TLVs are required
for the purpose of fair share AF appointment - Fair Share Sub-TLV and
AF Affinity VLAN Set Sub-TLV.
9.1 Fair Share Sub-TLV
Fair Share Sub-TLV is used to advertise the number of shares
configured in the RBridge. Number of shares is a two octet value.
When an RBridge advertises zero shares, it is not assigned any AF.
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Shares | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2 AF Affinity VLAN Set Sub-TLV
AF Affinity VLAN Set Sub-TLV is used to advertise the AF Affinity
VLAN set configured in an RBridge. It is a facsimile of the Enabled-
VLANs sub-TLV.
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Start VLAN ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VLAN bit-map....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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9.3 Partial VLANs Appointing Sub-TLV
As discussed in [RFC6439] section 2.2.3, the size of hello imposes a
limit on the distribution of AF info in AF Sub-TLV by the DRB. The
nature of the algorithm means that the AF appointment information
could be disjoint. If the number of VLANs on a shared link is too
high, all AF appointments cannot be accommodated in a single hello
using the start end mechanism of AF Sub-TLV. In such case, the DRB
should appoint one RBridge (possibly itself) as AF for all VLANs.
Alternatively, the AF information can be sent in a bitmap rather than
start-end mechanism as suggested in AF Sub-TLV. For this purpose the
Partial VLANs Appointing Sub-TLV suggested in Adaptive VLAN
Assignment draft [VlanAsn] can be used.
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10 Security Considerations
This document raises no new security issues for IS-IS.
11 IANA Considerations
This document suggests two additional Sub-TLV to Multi-Topology-Aware
Port Capability TLV apart from the reuse of Partial VLANs Appointing
Sub-TLV from Adaptive VLAN Assignment draft.
o Fair Share Sub-TLV
o AF Affinity VLAN Set Sub-TLV
12 References
12.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6325] R. Perlman, D. Eastlake, et al, "RBridges: Base Protocol
Specification", RFC 6325, July 2011.
[RFC6326] D. Eastlake, A. Banerjee, et al, "Transparent
Interconnection of Lots of Links (TRILL) Use of IS-IS", RFC
6326, July 2011.
[RFC6439] D. Eastlake, R. Perlman, et al, "Routing Bridges
(RBridges): Appointed Forwarders", RFC 6439, November 2011.
[RBisisb] D. Eastlake, A. Banerjee, et al, "Transparent
Interconnection of Lots of Links (TRILL) Use of IS-IS",
draft-eastlake-isis-rfc6326bis-09.txt, work in progress.
12.2 Informative References
[VlanAsn] M.Zhang and D.Zhang, "Adaptive VLAN Assignment for Data
Center RBridges", draft-zhang-trill-vlan-assign-04.txt,
work in progress.
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Authors' Addresses
Kesava Vijaya Krupakaran
Dell
Olympia Technology Park,
Guindy Chennai 600 032
Phone: +91 44 4220 8496
Email: Kesava_Vijaya_Krupak@Dell.com
Janardhanan Pathangi
Dell
Olympia Technology Park,
Guindy Chennai 600 032
Phone: +91 44 4220 8459
Email: Pathangi_Janardhanan@Dell.com
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