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Flexible Algorithms Bandwidth ConstraintsJuniper Networks Inc.Exora Business ParkBangaloreKA560103Indiashraddha@juniper.netJuniper Networks Inc.bwilliam@juniper.netJuniper Networks Inc.mrajesh@juniper.netOrangebruno.decraene@orange.com
Routing
SPRINGASIGP
Many networks configure the link metric relative to the link capacity.
High bandwidth traffic gets routed as per the link capacity. Flexible
algorithms provides mechanisms to create constraint based paths in IGP.
This draft documents a set of bandwidth related constraints to be used
in Flexible Algorithms.
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. High bandwidth traffic such as residential internet traffic
and machine to machine elephant flows benefit from using high
capacily links for the traffic.
Many network operators define link metric relative to the link capacity.
It may be useful to exclude the high bandwidth traffic from utilizing
links below certain capacity. A flex-algo
is defined as a set of parameters consisting of calculation-type,
metric-type and a set of constraints. It is very convenient to define a flex-algo
that uses bandwidth based metric-type which can be used for carrying high bandwidth
traffic.In this regard, it's useful to define additional metric-type and additional bandwidth
related constraints to simplify the operations. This document specifies a new metric-type to be used in flex-algo described
in section . Additional Flexible Algorithm Definition (FAD)
constraints defined in section .
defines mechanisms to automatically
calculate metric based on reference bandwidth and actual link bandwidth.
ISIS and OSPF advertise link metric in their respective link information.
Multiple types of metric are supported, IGP cost, te-metric defined in
and
and delay metric defined in and .
A brownfield network might have deployed legacy transport mechanisms using igp-cost
and te-metric which continue to run during migration period.
In this brownfield network if the operator wants to introduce
two Flex-Algos, one for delay metric
and another for bandwidth metric, a new metric-type
to carry bandwidth related metric, would be needed. The IGP cost and
te-metric may be already used by legacy applications and may not be
available to carry link bandwidth based metric.
This document defines a new metric called bandwidth metric. ISIS and OSPF
will advertise this new type of metric in their link information. This document also
defines a new metric-type called "bandwidth metric" in the FAD sub-TLV.
The ISIS Bandwidth Metric sub-TLV specifies the link metric based
on link bandwidth. Typically, this metric is assigned by a
network administrator.
The bandwidth metric sub-TLV is advertised in below TLVsTLV-22 (Extended IS reachability) [RFC5305]TLV-222 (MT-ISN) [RFC5120]TLV-23 (IS Neighbor Attribute) [RFC5311]TLV-223 (MT IS Neighbor Attribute) [RFC5311]TLV-141 (inter-AS reachability information) [RFC5316]
The bandwidth metric sub-TLV MUST be advertised only once. If there are multiple
bandwidth metric sub-TLVs in one or more received LSPDUs,
the first one MUST be used and
the subsequesnt ones MUST be ignored.
The Bandwidth Metric sub-TLV specifies the link metric based
on link bandwidth. Typically, this metric is assigned by a
network administrator.
The bandwidth metric sub-TLV is a sub-TLV of the
OSPF Link TLV and advertised in OSPF extended Link LSA
.
The Bandwidth Metric sub-TLV is TLV type TBD (IANA), and is four
octets in length.
The bandwidth metric sub-TLV MUST be advertised only once. If there are multiple
bandwidth metric sub-TLVs in a receieved Link TLV, the first one MUST be used and
the subsequent ones MUST be ignored.
It is useful to exclude Links having capacity lower than a minimum value from the
flex-algo topology that is designed to carry high bandwidth traffic.
This can be achieved by associating link affinity to the lower capacity
links and advertise exclude link constraint in the FAD for that link affinity.
This works well where the link capacity is constant. When a L3 link is
collection of L2 links (LAG/L2 Bundle) , the link bandwidth varies based on
the constituent link going up and down. The operator has to constantly monitor the
link capacity and assign appropriate link affinity on link capacity
changes beyond minimum value. In certain cases, the minimum link bandwidth required
may change based on the applications that use the high bandwidth Flex-Algo.
This document proposes a new exclude minimum bandwidth
constraint. When this constraint is advertised in a FAD, based on the
advertised link bandwidth, the link will be pruned from the flex-algo topology
if its below the FAD advertised Minimum bandwidth value.
Similarly, exclude maximum link delay constraint is also defined in this document.
Links may have the link delay measured dynamically and advertised in delay metric in IGP.
For usecases that deploy low latency flex-algo, may want to exclude links that have
delay more than a defined threshold.
ISIS Flex-algo Exclude Minimum Bandwidth sub-TLV (FAEMB) is a
sub-TLV of the ISIS FAD sub-TLV. It has the following format.
The FAEMB sub-TLV MUST appear only once in the FAD sub-TLV.
If it appears more than once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
The total link bandwidth as advertised by the sub-TLV 9 of the TLV
22/222/23/223/141 is compared against the Min bandwidth advertised in FAEMB sub-TLV.
If the link bandwidth is lower, the link is excluded from the Flex-algo topology
of the corresponding Flex-algo which advertised the FAEMB sub-TLV.
If a link does not have the link bandwidth advertised but the FAD contains this sub-TLV,
then that link MUST be excluded from the topology.
ISIS Flex-algo Exclude Maximum Delay sub-TLV (FAEMD) is a
sub-TLV of the ISIS FAD sub-TLV. It has the following format.
The FAEMD sub-TLV MUST appear only once in the FAD sub-TLV.
If it appears more than once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
The link delay .as advertised by the sub-TLV 33 of the TLV
22/222/23/223/141 is compared against the Max link delay advertised in FAEMD sub-TLV.
If the link delay value is higher, the link is excluded from the Flex-algo topology
of the corresponding Flex-algo which advertised the FAEMD sub-TLV.
If a link does not have the link delay advertised but the FAD contains this sub-TLV,
then that link MUST be excluded from the topology.
OSPF Flex-algo Bandwidth Exclusion sub-TLV (FAEMB) is a
sub-TLV of the OSPF FAD TLV. It has the following format.
The FAEMB sub-TLV MUST appear only once in the FAD sub-TLV.
If it appears more than once, the OSPF FAD TLV MUST be
ignored by the receiver.
The total link bandwidth as advertised by the sub-TLV 6 of the Extended Link TLV
of Extended Link Opaque LSA [RFC 7684]
is compared against the Min bandwidth advertised in FAEMB sub-TLV.
If the link bandwidth is lesser, the link is excluded from the Flex-algo topology
of the corresponding Flex-algo which advertised the FAEMB sub-TLV.
If a link does not have the link bandwidth advertised but the FAD contains this sub-TLV,
then that link MUST be excluded from the topology.
OSPF Flex-algo Exclude Maximum Delay sub-TLV (FAEMD) is a
sub-TLV of the OSPF FAD TLV. It has the following format.
The FAEMD sub-TLV MUST appear only once in the OSPF FAD TLV.
If it appears more than once, the OSPF FAD TLV MUST be
ignored by the receiver.
The link delay as advertised by the sub-TLV 27 of the Extended Link TLV
of Extended link opaque LSA [RFC 7684]
is compared against the Max delay advertised in FAEMD sub-TLV.
If the link delay value is higher, the link is excluded from the Flex-algo topology
of the corresponding Flex-algo which advertised the FAEMD sub-TLV.
If a link does not have the link delay advertised but the FAD contains this sub-TLV,
then that link MUST be excluded from the topology.
Networks which are designed to be highly regular and follow
uniform metric assignment may want to further
simplify the operations by automatically calculating the metric based on the
reference bandwidth or a staircase metric assignment based on bandwidth thresholds.
Based on the advertised rules, every node automatically calculates the link metric
of the links in the network before running SPF algorithm.
Based on the delay in receiving the link bandwidth
changes, there may be possibility of micro-loops which is no different from
IGP susceptibility to micro-loops during metric change. The micro-loop avoidance procedures
described in
can be used to avoid micro-loops when
the automatic metric calculation is deployed. In simple mode, the link bandwidth of a single Layer 3 link is used.
Two ways of automatic metric calculation is supported. 1. Based on reference bandwidth2. Staircase metric values based on bandwidth thresholdsSimple mode of metric calculation may not work well when there are
multiple parallel layer 3 interfaces between two nodes.
In the above diagram, there are two parallel links between B->C and C->D.
Lets assume the link bandwidth is uniform 10Gbps on all links. When Simple mode
of metric derivation is used,the metric is derived as 10 on all links.
Traffic will be load balanced beween B-> C->D and B->E->D. Since the bandwidth is
higher B->C->D path, the requirement is t be able to
assign smaller metric based on cumulative
metric for the parallel links.
In the interface group mode, every node
identifies the set of parallel links betweena pair of nodes based on
IGP link advertisements
and considers cumulative bandwidth of the parallel links while arriving
at the metric for the link. Two ways of automatic metric calculation is
supported for interface group mode as well.1. Based on reference bandwidth2. Staircase metric values based on bandwidth thresholdsThe cumulative metric of the parallel link is used and based on reference
bandwidth or staircase metric assignment method, metric value is derived.
The derived metric is assigned to each of the parallel link between nodes.
Interface group mode is useful for deployments that do not use L2 bundles.
The Flexible Algorithm Definition Reference Bandwidth Sub-TLV (FADRB Sub-TLV) is
a Sub-TLV of the ISIS FAD sub-TLV. It has the following format:
The ISIS FADRB Sub-TLV MUST NOT appear more then once in an ISIS FAD
sub-TLV. If it appears more then once, the ISIS FAD sub-TLV MUST be ignored
by the receiver.
The Flexible Algorithm Definition Threshold Bandwidth Sub-TLV (FADTB Sub-TLV) is
a Sub-TLV of the ISIS FAD sub-TLV. It has the following format:
The ISIS FADTB Sub-TLV MUST NOT appear more then once in an ISIS FAD
sub-TLV. If it appears more then once, the ISIS FAD sub-TLV MUST be ignored
by the receiver.
The Flexible Algorithm Definition Reference Bandwidth Sub-TLV (FADRB Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. It has the following format:
The OSPF FADRB Sub-TLV MUST NOT appear more then once in an OSPF FAD
TLV. If it appears more then once, the OSPF FAD TLV MUST be ignored
by the receiver.
The Flexible Algorithm Definition Threshold Bandwidth Sub-TLV (FADTB Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. It has the following format:
The OSPF FADTB Sub-TLV MUST NOT appear more then once in an OSPF FAD
TLV. If it appears more then once, the OSPF FAD TLV MUST be ignored
by the receiver. TBD Type: Suggested 3 (TBA) Description: Bandwidth metric Reference: This document Type: Suggested 6 (TBA) Description: ISIS Exclude Minimum Bandwidth sub-TLV Reference: This document Type: Suggested 7 (TBA) Description: ISIS Exclude Maximum Delay sub-TLV Reference: This document Type: Suggested 8 (TBA) Description: ISIS Reference Bandwidth sub-TLV Reference: This document Type: Suggested 9 (TBA) Description: ISIS Threshold metric sub-TLV Reference: This document Type: Suggested 6 (TBA) Description: OSPF Exclude Minimum Bandwidth sub-TLV Reference: This document Type: Suggested 7 (TBA) Description: OSPF Exclude Maximum Delay sub-TLV Reference: This document Type: Suggested 8 (TBA) Description: OSPF Reference Bandwidth sub-TLV Reference: This document Type: Suggested 9 (TBA) Description: OSPF Threshold metric sub-TLV Reference: This document Type: Suggested 45 (TBA) Description: Bandwidth metric Reference: This document Type: Suggested 45 (TBA) Description: Bandwidth metric Reference: This document Type: Suggested 45 (TBA) Description: Bandwidth metric Reference: This document Type: Suggested 45 (TBA) Description: Bandwidth metric Reference: This document Many thanks to Chris Bowers, Krzysztof Szarcowitz, Julian Lucek,
Ram Santhanakrishnan for discussions and inputs.1. Salih K AJuniper Networkssalih@juniper.net
&RFC2119;
&RFC5305;
&RFC3630;
&RFC7684;
&RFC8570;
&RFC7471;
&RFC5311;
&RFC5316;
&RFC5120;