draft-ietf-trill-mtu-negotiation-08v3.original   draft-ietf-trill-mtu-negotiation-08v3.txt 
INTERNET-DRAFT M. Zhang Network Working Group M. Zhang
Intended Status: Standards Track X. Zhang Internet-Draft X. Zhang
Updates: 6325, 7177, 7780 D. Eastlake Updates: 7780 (if approved) D. Eastlake
Huawei Intended status: Standards Track Huawei
R. Perlman Expires: February 3, 2018 R. Perlman
EMC EMC
S. Chatterjee S. Chatterjee
Cisco Cisco
Expires: February 3, 2018 August 2, 2017 August 2, 2017
Transparent Interconnection of Lots of Links (TRILL): Transparent Interconnection of Lots of Links (TRILL): MTU Negotiation
MTU Negotiation
draft-ietf-trill-mtu-negotiation-08.txt draft-ietf-trill-mtu-negotiation-08.txt
Abstract Abstract
The base IETF TRILL protocol has a TRILL campus-wide MTU feature, The base IETF TRILL protocol has a TRILL campus-wide MTU feature,
specified in RFC 6325 and RFC 7177, that assures that link state specified in RFC 6325 and RFC 7177, that assures that link state
changes can be successfully flooded throughout the campus while being changes can be successfully flooded throughout the campus while being
able to take advantage of a campus-wide capability to support jumbo able to take advantage of a campus-wide capability to support jumbo
packets. This document specifies recommended updates to that MTU packets. This document specifies recommended updates to that MTU
feature to take advantage, for appropriate link-local packets, of feature to take advantage, for appropriate link-local packets, of
link-local MTUs that exceed the TRILL campus MTU. In addition, it link-local MTUs that exceed the TRILL campus MTU. In addition, it
specifies an efficient algorithm for local MTU testing. This document specifies an efficient algorithm for local MTU testing. This
updates RFC 6325, updates RFC 7177, and updates RFC 7780. document updates RFC 6325, updates RFC 7177, and updates RFC 7780.
Status of this Memo Status of This Memo
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Copyright and License Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . . 3 1.1. Conventions used in this document . . . . . . . . . . . . 3
2. Link-Wide TRILL MTU Size . . . . . . . . . . . . . . . . . . . 3 2. Link-Wide TRILL MTU Size . . . . . . . . . . . . . . . . . . 3
2.1. Operations . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Operations . . . . . . . . . . . . . . . . . . . . . . . 4
3. Link MTU Size Testing . . . . . . . . . . . . . . . . . . . . . 6 3. Link MTU Size Testing . . . . . . . . . . . . . . . . . . . . 5
4. Refreshing Sz . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Refreshing Sz . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Relationship between Port MTU, Lz and Sz . . . . . . . . . . . 9 5. Relationship between Port MTU, Lz and Sz . . . . . . . . . . 9
6. LSP Synchronization . . . . . . . . . . . . . . . . . . . . . . 9 6. LSP Synchronization . . . . . . . . . . . . . . . . . . . . . 9
7. Recommendations for Traffic Link MTU Size Testing . . . . . . . 9 7. Recommendations for Traffic Link MTU Size Testing . . . . . . 9
8. Backwards Compatibility . . . . . . . . . . . . . . . . . . . . 10 8. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 10 9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Additions to Configuration . . . . . . . . . . . . . . . . . . 11 10. Additions to Configuration . . . . . . . . . . . . . . . . . 11
10.1. Per RBridge Configuration . . . . . . . . . . . . . . . . 11 10.1. Per RBridge Configuration . . . . . . . . . . . . . . . 11
10.2. Per RBridge Port Configuration . . . . . . . . . . . . . . 11 10.2. Per RBridge Port Configuration . . . . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . . 12 13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . . 12 13.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
[RFC6325] describes the way RBridges agree on the campus-wide minimum [RFC6325] describes the way RBridges agree on the campus-wide minimum
acceptable inter-RBridge MTU (Maximum Transmission Unit) size (called acceptable inter-RBridge MTU (Maximum Transmission Unit) size (called
"Sz") to ensure that link state flooding operates properly and all "Sz") to ensure that link state flooding operates properly and all
RBridges converge to the same link state. For the proper operation of RBridges converge to the same link state. For the proper operation
TRILL IS-IS, all RBridges format their LSPs to fit in Sz. of TRILL IS-IS, all RBridges format their LSPs to fit in Sz.
[RFC7177] diagrams the state transitions of an adjacency. If MTU [RFC7177] diagrams the state transitions of an adjacency. If MTU
testing is enabled, "Link MTU size is successfully tested" is part of testing is enabled, "Link MTU size is successfully tested" is part of
an event (event A6) causing the transition from "2-way" state to an event (event A6) causing the transition from "2-way" state to
"Report" state for an adjacency. This means the link MTU testing of "Report" state for an adjacency. This means the link MTU testing of
size X succeeds, and X is greater than or equal to Sz [RFC6325]. If size X succeeds, and X is greater than or equal to Sz [RFC6325]. If
this link cannot support an MTU of Sz, it will not be reported as this link cannot support an MTU of Sz, it will not be reported as
part of the campus topology. part of the campus topology.
In this document, a new RECOMMENDED link-wide minimum inter-RBridge In this document, a new RECOMMENDED link-wide minimum inter-RBridge
MTU size, Lz, is specified. As further discussed in Section 2, by MTU size, Lz, is specified. As further discussed in Section 2, by
calculating and using Lz as specified herein, link-scoped PDUs can be calculating and using Lz as specified herein, link-scoped PDUs can be
formatted greater than Sz, up to the link-wide minimum acceptable formatted greater than Sz, up to the link-wide minimum acceptable
inter-RBridge MTU size potentially improving the efficiency of link inter-RBridge MTU size potentially improving the efficiency of link
utilization and speeding link state convergence. utilization and speeding link state convergence.
An optional TRILL MTU size-testing algorithm is specified in Section An optional TRILL MTU size-testing algorithm is specified in
3 as an efficient method to update the old MTU testing method Section 3 as an efficient method to update the old MTU testing method
described in Section 4.3.2 of [RFC6325] and in [RFC7177]. The new MTU described in Section 4.3.2 of [RFC6325] and in [RFC7177]. The new
size testing method specified in this document is backward compatible MTU size testing method specified in this document is backward
with the old one. Multicasting the MTU-probes is recommended when compatible with the old one. Multicasting the MTU-probes is
there are multiple RBridges on a link responding to the probing with recommended when there are multiple RBridges on a link responding to
MTU-ack [RFC7177]. The testing method and rules of this document are the probing with MTU-ack [RFC7177]. The testing method and rules of
devised in a way to minimize the number of MTU probes for testing, this document are devised in a way to minimize the number of MTU
which therefore reduces the number of multicast packets for MTU probes for testing, which therefore reduces the number of multicast
testing. packets for MTU testing.
This document updates [RFC7780] as specified in Section 4. This document updates [RFC7780] as specified in Section 4.
1.1. Conventions used in this document 1.1. 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].
2. Link-Wide TRILL MTU Size 2. Link-Wide TRILL MTU Size
This document specifies a new value "Lz" for the minimum acceptable This document specifies a new value "Lz" for the minimum acceptable
inter-RBridge link MTU size on a local link. Link-wide Lz is the inter-RBridge link MTU size on a local link. Link-wide Lz is the
minimum Lz supported and agreed amongst all RBridges on a specific minimum Lz supported and agreed amongst all RBridges on a specific
link. If the link is usable, Lz will be greater than or equal to Sz. link. If the link is usable, Lz will be greater than or equal to Sz.
Some TRILL IS-IS PDUs are exchanged only between neighbors instead of Some TRILL IS-IS PDUs are exchanged only between neighbors instead of
the whole campus. They are confined by the link-wide Lz instead of the whole campus. They are confined by the link-wide Lz instead of
Sz. CSNPs and PSNPs are examples of such PDUs. These PDUs are Sz. CSNPs and PSNPs are examples of such PDUs. These PDUs are
exchanged only on the local link. (While TRILL IS-IS Hellos are also exchanged only on the local link. (While TRILL IS-IS Hellos are also
link local, they are always limited to 1470 bytes for robustness.) link local, they are always limited to 1470 bytes for robustness.)
[RFC7356] defines the PDUs which support flooding scopes in addition [RFC7356] defines the PDUs which support flooding scopes in addition
to area-wide scope and domain-wide scope. As specified in [RFC8139], to area-wide scope and domain-wide scope. As specified in [RFC8139],
RBridges support the Extended L1 Circuit Scoped (E-L1CS) flooding RBridges support the Extended L1 Circuit Scoped (E-L1CS) flooding
scope LSP (FS-LSP) [RFC7780]. The originatingSNPBufferSize for a port scope LSP (FS-LSP) [RFC7780]. The originatingSNPBufferSize for a
is the minimum of the following two quantities, but not less than port is the minimum of the following two quantities, but not less
1470 bytes: (1) the maximum MTU of the port and (2) the maximum LSP than 1470 bytes: (1) the maximum MTU of the port and (2) the maximum
size that the TRILL IS-IS implementation can handle. They use that LSP size that the TRILL IS-IS implementation can handle. They use
flooding to exchange their maximum supported value of "Lz". The that flooding to exchange their maximum supported value of "Lz". The
smallest value of the Lz advertised by the RBridges on a link, but smallest value of the Lz advertised by the RBridges on a link, but
not less than Sz, is the link-wide Lz. An RBridge on a local link not less than Sz, is the link-wide Lz. An RBridge on a local link
will be able to tell which other RBridges on that link support E-L1CS will be able to tell which other RBridges on that link support E-L1CS
FS-LSPs because, as required by [RFC7780], all RBridges include the FS-LSPs because, as required by [RFC7780], all RBridges include the
Scoped Flooding Support TLV [RFC7356] in their TRILL Hellos. Scoped Flooding Support TLV [RFC7356] in their TRILL Hellos.
The maximum sized level 1 link-local PDU, such as PSNP or CSNP, which The maximum sized level 1 link-local PDU, such as PSNP or CSNP, which
may be generated by a system is controlled by the value of the may be generated by a system is controlled by the value of the
management parameter originatingL1SNPBufferSize. This value management parameter originatingL1SNPBufferSize. This value
determines Lz. The TRILL APPsub-TLV shown in Figure 2.1 SHOULD be determines Lz. The TRILL APPsub-TLV shown in Figure 2.1 SHOULD be
included in a TRILL GENINFO TLV [RFC7357] in an E-L1CS FS-LSP included in a TRILL GENINFO TLV [RFC7357] in an E-L1CS FS-LSP
fragment zero. If it is missing from a fragment zero E-L1CS FS-LSP or fragment zero. If it is missing from a fragment zero E-L1CS FS-LSP
there is no fragment zero E-L1CS FS-LSP, it is assumed that its or there is no fragment zero E-L1CS FS-LSP, it is assumed that its
originating IS is implicitly advertising its originatingSNPBufferSize originating IS is implicitly advertising its originatingSNPBufferSize
value as Sz octets. value as Sz octets.
E-L1CS FS-LSPs are link-local and can also be sent up to Lz in size E-L1CS FS-LSPs are link-local and can also be sent up to Lz in size
but, for robustness, E-L1CS FS-LSP fragment zero MUST NOT exceed 1470 but, for robustness, E-L1CS FS-LSP fragment zero MUST NOT exceed 1470
bytes. bytes.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = tbd | (2 byte) | Type = tbd | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length = 2 | (2 byte) | Length = 2 | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| originatingSNPBufferSize | (2 byte) | originatingSNPBufferSize | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2.1: The originatingSNPBufferSize TLV. Figure 1: The originatingSNPBufferSize TLV.
Type: set to originatingSNPBufferSize APPsubTLV (TRILL APPsub-TLV Type: set to originatingSNPBufferSize APPsubTLV (TRILL APPsub-TLV
type tbd). Two bytes because this APPsub-TLV appears in an Extended type tbd). Two bytes because this APPsub-TLV appears in an Extended
TLV [RFC7356]. TLV [RFC7356].
Length: set to 2. Length: set to 2.
originatingSNPBufferSize: the local value of originatingSNPBufferSize: the local value of
originatingL1SNPBufferSize as an unsigned integer, limited in the originatingL1SNPBufferSize as an unsigned integer, limited in the
range from 1470 to 65,535 bytes. (A value less than 1470 will be range from 1470 to 65,535 bytes. (A value less than 1470 will be
ignored.) ignored.)
2.1. Operations 2.1. Operations
Lz MAY be reported using an originatingSNPBufferSize TLV that occurs Lz MAY be reported using an originatingSNPBufferSize TLV that occurs
in fragment zero of the RBridge's E-L1CS FS-LSP. An in fragment zero of the RBridge's E-L1CS FS-LSP. An
originatingSNPBufferSize APPsub-TLV occurring in any other fragment originatingSNPBufferSize APPsub-TLV occurring in any other fragment
is ignored. If more than one originatingSNPBufferSize APPsub-TLV is ignored. If more than one originatingSNPBufferSize APPsub-TLV
occurs in fragment zero, the one advertising the smallest value for occurs in fragment zero, the one advertising the smallest value for
originatingSNPBufferSize, but not less than 1470 bytes, is used. originatingSNPBufferSize, but not less than 1470 bytes, is used.
Lz:1800 Lz:1800 Lz:1800 Lz:1800
+---+ | +---+ +---+ | +---+
|RB1|(2000)---|---(2000)|RB2| |RB1|(2000)---|---(2000)|RB2|
+---+ | +---+ +---+ | +---+
| |
Lz:1800 | Lz:1800 |
+---+ +--+ +---+ +--+
|RB3|(2000)---(1700)|B1| |RB3|(2000)---(1700)|B1|
+---+ +--+ +---+ +--+
| |
Figure 2.2: Link-wide Lz = 1800 v.s. tested link MTU size = 1700 Figure 2: Link-wide Lz = 1800 v.s. tested link MTU size = 1700
Even if all RBridges on a specific link have reached consensus on the Even if all RBridges on a specific link have reached consensus on the
value of link-wide Lz based on advertised originatingSNPBufferSize, value of link-wide Lz based on advertised originatingSNPBufferSize,
it does not mean that these RBridges can safely exchange PDUs between it does not mean that these RBridges can safely exchange PDUs between
each other. Figure 2.2 shows such a corner case. RB1, RB2 and RB3 are each other. Figure 2.2 shows such a corner case. RB1, RB2 and RB3
three RBridges on the same link and their Lz is 1800, so the link- are three RBridges on the same link and their Lz is 1800, so the
wide Lz of this link is 1800. There is an intermediate bridge (say link- wide Lz of this link is 1800. There is an intermediate bridge
B1) between RB2 and RB3 whose port MTU size is 1700. If RB2 sends (say B1) between RB2 and RB3 whose port MTU size is 1700. If RB2
PDUs formatted in chunks of size 1800, it will be discarded by B1. sends PDUs formatted in chunks of size 1800, it will be discarded by
B1.
Therefore the link MTU size SHOULD be tested. After the link MTU size Therefore the link MTU size SHOULD be tested. After the link MTU
of an adjacency is successfully tested, those link-local PDUs such as size of an adjacency is successfully tested, those link-local PDUs
CSNPs, PSNPs and E-L1CS FS-LSPs will be formatted no greater than the such as CSNPs, PSNPs and E-L1CS FS-LSPs will be formatted no greater
tested link MTU size and will be safely transmitted on this link. than the tested link MTU size and will be safely transmitted on this
link.
As for Sz, RBridges continue to propagate their As for Sz, RBridges continue to propagate their
originatingL1LSPBufferSize across the campus through the originatingL1LSPBufferSize across the campus through the
advertisement of LSPs as defined in Section 4.3.2 of [RFC6325]. The advertisement of LSPs as defined in Section 4.3.2 of [RFC6325]. The
smallest value of Sz advertised by any RBridge, but not less than smallest value of Sz advertised by any RBridge, but not less than
1470, will be deemed as Sz. Each RBridge formats their "campus-wide" 1470, will be deemed as Sz. Each RBridge formats their "campus-wide"
PDUs, for example LSPs, not greater than what they determine as Sz. PDUs, for example LSPs, not greater than what they determine as Sz.
3. Link MTU Size Testing 3. Link MTU Size Testing
[RFC7177] defines the event A6 as including "MTU test is successful" [RFC7177] defines the event A6 as including "MTU test is successful"
if the MTU testing is enabled. As described in Section 4.3.2 of if the MTU testing is enabled. As described in Section 4.3.2 of
[RFC6325], this is a combination of the following event and [RFC6325], this is a combination of the following event and
condition. condition.
Event: The link MTU size has been tested. Event: The link MTU size has been tested.
Condition: The link can support Sz. Condition: The link can support Sz.
This condition can be efficiently tested by the following "Binary This condition can be efficiently tested by the following "Binary
Search Algorithm" and rules. This updates [RFC7177] and [RFC6325]. Search Algorithm" and rules. This updates [RFC7177] and [RFC6325].
x, lowerBound, and upperBound are local integer variables. The MTU- x, lowerBound, and upperBound are local integer variables. The MTU-
probe and MTU-ack PDUs are specified in Section 3 of [RFC7176]. probe and MTU-ack PDUs are specified in Section 3 of [RFC7176].
Step 0: RB1 sends an MTU-probe padded to the size of link-wide Lz. Step 0: RB1 sends an MTU-probe padded to the size of link-wide Lz.
1) If RB1 successfully receives the MTU-ack from RB2 to the probe of 1) If RB1 successfully receives the MTU-ack from RB2 to the probe of
the value of link-wide Lz within k tries (where k is a the value of link-wide Lz within k tries (where k is a
configurable parameter whose default is 3. One Round Trip Time configurable parameter whose default is 3. One Round Trip Time
(RTT) between the two adjacent RBridges is RECOMMENDED to be used (RTT) between the two adjacent RBridges is RECOMMENDED to be used
as the minimum interval between two successive probes. Note that as the minimum interval between two successive probes. Note that
RTT estimation is out of the scope for this document. If operators RTT estimation is out of the scope for this document. If
cannot not estimate the RTT, the default value 5-millisecond operators cannot not estimate the RTT, the default value
should be assumed.), link MTU size is set to the size of link-wide 5-millisecond should be assumed.), link MTU size is set to the
Lz and stop. size of link-wide Lz and stop.
2) RB1 tries to send an MTU-probe padded to the size 1470. 2) RB1 tries to send an MTU-probe padded to the size 1470.
a) If RB1 fails to receive an MTU-ack from RB2 after k tries (An a) If RB1 fails to receive an MTU-ack from RB2 after k tries (An
MTU-ack should be considered to have failed two RTT after the MTU-ack should be considered to have failed two RTT after the
probe is sent out.), RB1 sets the "failed minimum MTU test" probe is sent out.), RB1 sets the "failed minimum MTU test" flag
flag for RB2 in RB1's Hello and stop. for RB2 in RB1's Hello and stop.
b) Link MTU size is set to 1470, lowerBound is set to 1470, b) Link MTU size is set to 1470, lowerBound is set to 1470,
upperBound is set to the link-wide Lz, x is set to [(lowerBound upperBound is set to the link-wide Lz, x is set to [(lowerBound
+ upperBound)/2], rounded down to the nearest integer. + upperBound)/2], rounded down to the nearest integer.
Step 1: RB1 tries to send an MTU-probe padded to the size x. Step 1: RB1 tries to send an MTU-probe padded to the size x.
1) If RB1 fails to receive an MTU-ack from RB2 after k tries: 1) If RB1 fails to receive an MTU-ack from RB2 after k tries:
upperBound is set to x-1 and x is set to [(lowerBound + upperBound is set to x-1 and x is set to [(lowerBound +
upperBound)/2], rounded down to the nearest integer. upperBound)/2], rounded down to the nearest integer.
2) If RB1 receives an MTU-ack to a probe of size x from RB2: 2) If RB1 receives an MTU-ack to a probe of size x from RB2:
link MTU size is set to x, lowerBound is set to x and x is set link MTU size is set to x, lowerBound is set to x and x is set
to [(lowerBound + upperBound)/2], rounded down to the nearest to [(lowerBound + upperBound)/2], rounded down to the nearest
integer. If lowerBound equals upperBound-1 then x is set to integer. If lowerBound equals upperBound-1 then x is set to
upperBound. upperBound.
3) If lowerBound >= upperBound or Step 1 has been repeated n times 3) If lowerBound >= upperBound or Step 1 has been repeated n times
(where n is a configurable parameter whose default value is 5), (where n is a configurable parameter whose default value is 5),
stop. stop.
4) Repeat Step 1. 4) Repeat Step 1.
After the testing, the two connected RBridges agree on the value of After the testing, the two connected RBridges agree on the value of
the link MTU size. MTU testing is only done in the Designated VLAN the link MTU size. MTU testing is only done in the Designated VLAN
[RFC7177]. Since the execution of the above algorithm can be resource [RFC7177]. Since the execution of the above algorithm can be
consuming, it is RECOMMENDED that the Designated RBRidge (DRB resource consuming, it is RECOMMENDED that the Designated RBRidge
[RFC7177]) take the responsibility to do the testing. Multicast MTU- (DRB [RFC7177]) take the responsibility to do the testing. Multicast
probes are used instead of unicast when multiple RBridges are desired MTU- probes are used instead of unicast when multiple RBridges are
to respond with an MTU-ack on the link. The Binary Search Algorithm desired to respond with an MTU-ack on the link. The Binary Search
given here is a way to minimize the probing attempts; it reduces the Algorithm given here is a way to minimize the probing attempts; it
number of multicast packets for MTU-probing. reduces the number of multicast packets for MTU-probing.
The following rules are designed to determine whether the The following rules are designed to determine whether the
aforementioned "Condition" holds. aforementioned "Condition" holds.
RBridges have figured out the upper bound and lower bound for the RBridges have figured out the upper bound and lower bound for the
link MTU size from the execution of the above algorithm. If Sz is link MTU size from the execution of the above algorithm. If Sz is
smaller than the lower bound or greater than the upper bound, smaller than the lower bound or greater than the upper bound,
RBridges can directly judge whether the link supports Sz without MTU- RBridges can directly judge whether the link supports Sz without MTU-
probing. probing.
(a) If "lowerBound" >= Sz. This link can support Sz. (a) If "lowerBound" >= Sz. This link can support Sz.
(b) Else if "upperBound" <= Sz. This link cannot support Sz. (b) Else if "upperBound" <= Sz. This link cannot support Sz.
Otherwise, RBridges SHOULD test whether the link can support Sz as in Otherwise, RBridges SHOULD test whether the link can support Sz as in
item (c) below. If they do not, the only safe assumption will be that item (c) below. If they do not, the only safe assumption will be
the link cannot support Sz. This assumption, without testing, might that the link cannot support Sz. This assumption, without testing,
rule out the use of a link that can, in fact, handle packets up to might rule out the use of a link that can, in fact, handle packets up
Sz. In the worst case, this might result in unnecessary network to Sz. In the worst case, this might result in unnecessary network
partition. partition.
(c) "lowerBound" < Sz < "upperBound". RBridges probe the link with (c) "lowerBound" < Sz < "upperBound". RBridges probe the link with
MTU-probe messages padded to Sz. If an MTU-ack is received within MTU-probe messages padded to Sz. If an MTU-ack is received
k tries, this link can support Sz. Otherwise, this link cannot within k tries, this link can support Sz. Otherwise, this link
support Sz. Through this test, the lower bound and upper bound of cannot support Sz. Through this test, the lower bound and upper
link MTU size can be updated accordingly. bound of link MTU size can be updated accordingly.
4. Refreshing Sz 4. Refreshing Sz
RBridges may join or leave the campus, which may change Sz. RBridges may join or leave the campus, which may change Sz.
1) Joining 1) Joining
a) When a new RBridge joins the campus and its a) When a new RBridge joins the campus and its
originatingL1LSPBufferSize is smaller than current Sz, originatingL1LSPBufferSize is smaller than current Sz, reporting
reporting its originatingL1LSPBufferSize in its LSPs will cause its originatingL1LSPBufferSize in its LSPs will cause other
other RBridges decrease their Sz. Then any LSP greater than the RBridges decrease their Sz. Then any LSP greater than the reduced
reduced Sz MUST be split and/or the LSP contents in the campus Sz MUST be split and/or the LSP contents in the campus MUST be
MUST be otherwise redistributed so that no LSP is greater than otherwise redistributed so that no LSP is greater than the new Sz.
the new Sz.
b) If the joining RBridge's originatingL1LSPBufferSize is equal to b) If the joining RBridge's originatingL1LSPBufferSize is equal to
or bigger than current Sz, reporting its or bigger than current Sz, reporting its
originatingL1LSPBufferSize will not change Sz. originatingL1LSPBufferSize will not change Sz.
2) Leaving 2) Leaving
a) From the specification of the Joining process, we know it's a) From the specification of the Joining process, we know it's
non-applicable that an RBridge leaves the campus while its non-applicable that an RBridge leaves the campus while its
originatingL1LSPBufferSize is smaller than Sz. originatingL1LSPBufferSize is smaller than Sz.
b) When an RBridge leaves the campus and its b) When an RBridge leaves the campus and its
originatingL1LSPBufferSize equals to Sz, its LSPs are purged originatingL1LSPBufferSize equals to Sz, its LSPs are purged
from the remaining campus after reaching MaxAge [IS-IS]. Sz MAY from the remaining campus after reaching MaxAge [IS-IS]. Sz
be recalculated and MAY increase. In other words, while in most MAY be recalculated and MAY increase. In other words, while in
cases RB1 ignores link state information for IS-IS unreachable most cases RB1 ignores link state information for IS-IS
RBridge RB2 [RFC7780], originatingL1LSPBufferSize is unreachable RBridge RB2 [RFC7780], originatingL1LSPBufferSize
meaningful. Its value, even from IS-IS unreachable RBridges, is is meaningful. Its value, even from IS-IS unreachable
used in determining Sz. This updates [RFC7780]. RBridges, is used in determining Sz. This updates [RFC7780].
c) When an RBrige leaves the campus and its c) When an RBrige leaves the campus and its
originatingL1LSPBufferSize is greater than Sz, this will not originatingL1LSPBufferSize is greater than Sz, this will not
update Sz since Sz is determined by another RBridge with update Sz since Sz is determined by another RBridge with
smaller originatingL1LSPBufferSize. smaller originatingL1LSPBufferSize.
Frequent LSP "re-sizing" is harmful to the stability of the TRILL Frequent LSP "re-sizing" is harmful to the stability of the TRILL
campus, so, to avoid this, upward resizing SHOULD be dampened. When campus, so, to avoid this, upward resizing SHOULD be dampened. When
an upward resizing event is noticed by an RBridge, it is RECOMMENDED an upward resizing event is noticed by an RBridge, it is RECOMMENDED
that a timer be set at that RBridge. This is a configurable that a timer be set at that RBridge. This is a configurable
parameter, LSPresizeTime, whose default value is 300 seconds. Before parameter, LSPresizeTime, whose default value is 300 seconds. Before
this timer expires, all subsequent upward resizing will be dampened this timer expires, all subsequent upward resizing will be dampened
(ignored). Of course, in a well-configured campus with all RBridges (ignored). Of course, in a well-configured campus with all RBridges
configured to have the same originatingL1LSPBufferSize, no resizing configured to have the same originatingL1LSPBufferSize, no resizing
will be necessary. It does not matter if different RBridges have will be necessary. It does not matter if different RBridges have
different dampening timers or some RBridges re-size upward more different dampening timers or some RBridges re-size upward more
quickly than others. quickly than others.
If the refreshed Sz is smaller than the lower bound or greater than If the refreshed Sz is smaller than the lower bound or greater than
the upper bound of the tested link MTU size, the resource consuming the upper bound of the tested link MTU size, the resource consuming
link MTU size testing can be avoided according to rule (a) or (b) link MTU size testing can be avoided according to rule (a) or (b)
specified in Section 3. Otherwise, RBridges test the link MTU size specified in Section 3. Otherwise, RBridges test the link MTU size
according to rule (c). according to rule (c).
5. Relationship between Port MTU, Lz and Sz 5. Relationship between Port MTU, Lz and Sz
When the port MTU of an RBridge is smaller than the local When the port MTU of an RBridge is smaller than the local
originatingL1SNPBufferSize of an RBridge (an inconsistent originatingL1SNPBufferSize of an RBridge (an inconsistent
configuration), that port SHOULD be disabled since, in any case, an configuration), that port SHOULD be disabled since, in any case, an
adjacency cannot be formed through such a port. On the other hand, adjacency cannot be formed through such a port. On the other hand,
when an RBridge receives an LSP or E-L1CS FS-LSP with size greater when an RBridge receives an LSP or E-L1CS FS-LSP with size greater
than the link-wide Lz or Sz but not greater than its port MTU size, than the link-wide Lz or Sz but not greater than its port MTU size,
this LSP is processed normally. If the size of an LSP is greater than this LSP is processed normally. If the size of an LSP is greater
the MTU size of a port over which it is to be propagated, this LSP than the MTU size of a port over which it is to be propagated, this
MUST NOT be sent over the port and an LSPTooLargeToPropagate alarm LSP MUST NOT be sent over the port and an LSPTooLargeToPropagate
shall be generated [IS-IS]. alarm shall be generated [IS-IS].
6. LSP Synchronization 6. LSP Synchronization
An RBridge participates in LSP synchronization on a link as soon as An RBridge participates in LSP synchronization on a link as soon as
it has at least one adjacency on that link that has advanced to at it has at least one adjacency on that link that has advanced to at
least the 2-Way state [RFC7177]. On a LAN link, CSNP and PSNP PDUs least the 2-Way state [RFC7177]. On a LAN link, CSNP and PSNP PDUs
are used for synchronization. On a point-to-point link, only PSNP are are used for synchronization. On a point-to-point link, only PSNP
used. are used.
The CSNPs and PSNPs can be formatted in chunks of size at most the The CSNPs and PSNPs can be formatted in chunks of size at most the
link-wide Lz but are processed normally if received larger than that link-wide Lz but are processed normally if received larger than that
size. Since the link MTU size may not have been tested in the 2-Way size. Since the link MTU size may not have been tested in the 2-Way
state, link-wide Lz may be greater than the supported link MTU size. state, link-wide Lz may be greater than the supported link MTU size.
In that case, a CSNP or PSNP may be discarded. After the link MTU In that case, a CSNP or PSNP may be discarded. After the link MTU
size is successfully tested, RBridges will begin to format these PDUs size is successfully tested, RBridges will begin to format these PDUs
in the size no greater than that MTU, therefore these PDUs will in the size no greater than that MTU, therefore these PDUs will
eventually get through. eventually get through.
Note that the link MTU size is frequently greater than Sz. Link-local Note that the link MTU size is frequently greater than Sz. Link-
PDUs are limited in the size by the link MTU size rather than Sz, local PDUs are limited in the size by the link MTU size rather than
which, when Lz is greater than Sz, promises a reduction in the number Sz, which, when Lz is greater than Sz, promises a reduction in the
of PDUs and a faster LSP synchronization process. number of PDUs and a faster LSP synchronization process.
7. Recommendations for Traffic Link MTU Size Testing 7. Recommendations for Traffic Link MTU Size Testing
Sz and link-wide Lz are used to limit the size of most TRILL IS-IS Sz and link-wide Lz are used to limit the size of most TRILL IS-IS
PDUs. They are different from the MTU size restricting the size of PDUs. They are different from the MTU size restricting the size of
TRILL Data packets. The size of a TRILL Data packet is restricted by TRILL Data packets. The size of a TRILL Data packet is restricted by
the physical MTU of the ports and links the packet traverses. It is the physical MTU of the ports and links the packet traverses. It is
possible that a TRILL Data packet successfully gets through the possible that a TRILL Data packet successfully gets through the
campus but its size is greater than Sz or link-wide Lz values. campus but its size is greater than Sz or link-wide Lz values.
The algorithm defined for link MTU size testing can also be used in The algorithm defined for link MTU size testing can also be used in
TRILL traffic MTU size testing; in that case the link-wide Lz used in TRILL traffic MTU size testing; in that case the link-wide Lz used in
that algorithm is replaced by the port MTU of the RBridge sending MTU that algorithm is replaced by the port MTU of the RBridge sending MTU
probes. The successfully tested size X MAY be advertised as an probes. The successfully tested size X MAY be advertised as an
attribute of this link using MTU sub-TLV defined in [RFC7176]. attribute of this link using MTU sub-TLV defined in [RFC7176].
Unlike RBridges, end stations do not participate in the exchange of Unlike RBridges, end stations do not participate in the exchange of
TRILL IS-IS PDUs; therefore, they cannot grasp the traffic link MTU TRILL IS-IS PDUs; therefore, they cannot grasp the traffic link MTU
size from a TRILL campus automatically. An operator may collect these size from a TRILL campus automatically. An operator may collect
values using network management tools such as TRILL ping or these values using network management tools such as TRILL ping or
TraceRoute. Then, the path MTU can be set as the smallest tested link TraceRoute. Then, the path MTU can be set as the smallest tested
MTU on this path; and end stations should not generate frames that, link MTU on this path; and end stations should not generate frames
when encapsulated as TRILL Data packets, exceed this path MTU. that, when encapsulated as TRILL Data packets, exceed this path MTU.
8. Backwards Compatibility 8. Backwards Compatibility
There can be a mixture of Lz-ignorant and Lz-aware RBridges on a There can be a mixture of Lz-ignorant and Lz-aware RBridges on a
link. This will act properly although, it may not be as efficient as link. This will act properly although, it may not be as efficient as
it would be if all RBridges on the link are Lz-aware. it would be if all RBridges on the link are Lz-aware.
For an Lz-ignorant RBridge, TRILL IS-IS PDUs are always formatted not For an Lz-ignorant RBridge, TRILL IS-IS PDUs are always formatted not
greater than Sz. Lz-aware RBridges as receivers can handle these PDUs greater than Sz. Lz-aware RBridges as receivers can handle these
since they cannot be greater than the link-wide Lz. PDUs since they cannot be greater than the link-wide Lz.
For an Lz-aware RBridge, in the case that link-wide Lz is greater For an Lz-aware RBridge, in the case that link-wide Lz is greater
than Sz, larger link-local TRILL IS-IS PDUs can be sent out to gain than Sz, larger link-local TRILL IS-IS PDUs can be sent out to gain
efficiencies. Lz-ignorant RBridges as receivers will have no problem efficiencies. Lz-ignorant RBridges as receivers will have no problem
handling them since the originatingL1LSPBufferSize value of these handling them since the originatingL1LSPBufferSize value of these
RBridges had been tested and the link-wide Lz is not greater than RBridges had been tested and the link-wide Lz is not greater than
that value. that value.
An Lz-ignorant RBridge might not support the link MTU testing An Lz-ignorant RBridge might not support the link MTU testing
algorithm defined in Section 3 but could be using some algorithm just algorithm defined in Section 3 but could be using some algorithm just
to test for Sz MTU on the link. In any case, if an RBridge per to test for Sz MTU on the link. In any case, if an RBridge per
[RFC6325] receives an MTU-probe, it MUST respond with an MTU-ack [RFC6325] receives an MTU-probe, it MUST respond with an MTU-ack
padded to the same size as the MTU-probe. padded to the same size as the MTU-probe.
9. Security Considerations 9. Security Considerations
This document raises no significant new security issues for TRILL. In This document raises no significant new security issues for TRILL.
TRILL, RBridges are generally considered to be trusted devices. In TRILL, RBridges are generally considered to be trusted devices.
Protection against forged TRILL IS-IS PDUs, including forged Hellos Protection against forged TRILL IS-IS PDUs, including forged Hellos
containing originatingSNPBufferSize APP-subTLVs, can be obtained containing originatingSNPBufferSize APP-subTLVs, can be obtained
through IS-IS PDU cryptographic authentication [RFC5310]. The worst through IS-IS PDU cryptographic authentication [RFC5310]. The worst
that an RBridge can do by reporting an erroneous that an RBridge can do by reporting an erroneous
originatingSNPBufferSize is reduce Lz to Sz and thus make unavailable originatingSNPBufferSize is reduce Lz to Sz and thus make unavailable
the optimization of being able to use link MTUs that exceed the the optimization of being able to use link MTUs that exceed the
campus wide MTU for link local TRILL IS-IS PDUs. campus wide MTU for link local TRILL IS-IS PDUs.
For general and adjacency related TRILL security considerations, see For general and adjacency related TRILL security considerations, see
[RFC6325] and [RFC7177]. [RFC6325] and [RFC7177].
10. Additions to Configuration 10. Additions to Configuration
Implementation of the features specified in this document adds two Implementation of the features specified in this document adds two
RBridge configuration parameters as follows: RBridge configuration parameters as follows:
10.1. Per RBridge Configuration 10.1. Per RBridge Configuration
Each RBridge implementing the RECOMMENDED LSP re-sizing damping Each RBridge implementing the RECOMMENDED LSP re-sizing damping
strategy specified in Section 4 has an LSPresizeTime parameter that strategy specified in Section 4 has an LSPresizeTime parameter that
is an integer in the range of 0-65,535 which defaults to 300. It is is an integer in the range of 0-65,535 which defaults to 300. It is
the number of seconds for which an RBridge determines that Sz has the number of seconds for which an RBridge determines that Sz has
increased before it will create any LSP or E-L1FS FS-LSP fragments. increased before it will create any LSP or E-L1FS FS-LSP fragments.
10.2. Per RBridge Port Configuration 10.2. Per RBridge Port Configuration
Each RBridge port on which the calculation and use of Lz is Each RBridge port on which the calculation and use of Lz is
implemented has an originatingL1SNPBufferSize parameter that is an implemented has an originatingL1SNPBufferSize parameter that is an
integer in the range of 1,470-65,535. This parameter defaults to the integer in the range of 1,470-65,535. This parameter defaults to the
minimum of the size that the port can accommodate and the size link- minimum of the size that the port can accommodate and the size link-
local IS-IS PDU that the TRILL implementation can accommodate. local IS-IS PDU that the TRILL implementation can accommodate.
11. IANA Considerations 11. IANA Considerations
IANA is requested to assign a new APPsub-TLV number from the range IANA is requested to assign a new APPsub-TLV number from the range
less than 256 in the "TRILL APPsub-TLV Types under IS-IS TLV 251 less than 256 in the "TRILL APPsub-TLV Types under IS-IS TLV 251
Application Identifier 1" registry for the TRILL Application Identifier 1" registry for the TRILL
originatingSNPBufferSize sub-TLV defined in Section 2 of this originatingSNPBufferSize sub-TLV defined in Section 2 of this
document. The entry is as follows: document. The entry is as follows:
Type Name Reference Type Name Reference
---- ------------------------ --------------- ---- ------------------------ ---------------
tbd originatingSNPBufferSize [this document] tbd originatingSNPBufferSize [this document]
12. Acknowledgements 12. Acknowledgements
Authors would like to thank the comments and suggestions from Vishwas Authors would like to thank the comments and suggestions from Vishwas
Manral. Manral.
13. References 13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 13.1. Normative References
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
and M. Fanto, "IS-IS Generic Cryptographic Authentication", Requirement Levels", BCP 14, RFC 2119,
RFC 5310, DOI 10.17487/RFC5310, February 2009, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc5310>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A. [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
Ghanwani, "Routing Bridges (RBridges): Base Protocol and M. Fanto, "IS-IS Generic Cryptographic
Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011, Authentication", RFC 5310, DOI 10.17487/RFC5310, February
<http://www.rfc-editor.org/info/rfc6325>. 2009, <http://www.rfc-editor.org/info/rfc5310>.
[RFC7177] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and [RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
V. Manral, "Transparent Interconnection of Lots of Links Ghanwani, "Routing Bridges (RBridges): Base Protocol
(TRILL): Adjacency", RFC 7177, DOI 10.17487/RFC7177, May Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
2014, <http://www.rfc-editor.org/info/rfc7177>. <http://www.rfc-editor.org/info/rfc6325>.
[RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt, D., [RFC7177] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
and A. Banerjee, "Transparent Interconnection of Lots of V. Manral, "Transparent Interconnection of Lots of Links
Links (TRILL) Use of IS-IS", RFC 7176, DOI (TRILL): Adjacency", RFC 7177, DOI 10.17487/RFC7177, May
10.17487/RFC7176, May 2014, <http://www.rfc- 2014, <http://www.rfc-editor.org/info/rfc7177>.
editor.org/info/rfc7176>.
[RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding [RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
Scope Link State PDUs (LSPs)", RFC 7356, DOI D., and A. Banerjee, "Transparent Interconnection of Lots
10.17487/RFC7356, September 2014, <http://www.rfc- of Links (TRILL) Use of IS-IS", RFC 7176,
editor.org/info/rfc7356>. DOI 10.17487/RFC7176, May 2014,
<http://www.rfc-editor.org/info/rfc7176>.
[RFC7780] Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A., [RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
Ghanwani, A., and S. Gupta, "Transparent Interconnection of Scope Link State PDUs (LSPs)", RFC 7356,
Lots of Links (TRILL): Clarifications, Corrections, and DOI 10.17487/RFC7356, September 2014,
Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016, <http://www.rfc-editor.org/info/rfc7356>.
<http://www.rfc-editor.org/info/rfc7780>.
[RFC7357] Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O. [RFC7780] Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
Stokes, "Transparent Interconnection of Lots of Links Ghanwani, A., and S. Gupta, "Transparent Interconnection
(TRILL): End Station Address Distribution Information of Lots of Links (TRILL): Clarifications, Corrections, and
(ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357, Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
September 2014, <http://www.rfc-editor.org/info/rfc7357>. <http://www.rfc-editor.org/info/rfc7780>.
13.2. Informative References [RFC7357] Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
Stokes, "Transparent Interconnection of Lots of Links
(TRILL): End Station Address Distribution Information
(ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357,
September 2014, <http://www.rfc-editor.org/info/rfc7357>.
[IS-IS] International Organization for Standardization, 13.2. Informative References
"Information technology -- Telecommunications and
information exchange between systems -- Intermediate System
to Intermediate System intra-domain routeing information
exchange protocol for use in conjunction with the protocol
for providing the connectionless-mode network service (ISO
8473)", ISO/IEC 10589:2002, Second Edition, November 2002.
[RFC8139] Eastlake 3rd, D., Li, Y., Umair, M., Banerjee, A., and F. [RFC8139] Eastlake 3rd, D., Li, Y., Umair, M., Banerjee, A., and F.
Hu, "Transparent Interconnection of Lots of Links (TRILL): Hu, "Transparent Interconnection of Lots of Links (TRILL):
Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139, June Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139,
2017, <http://www.rfc-editor.org/info/rfc8139>. June 2017, <http://www.rfc-editor.org/info/rfc8139>.
Author's Addresses Authors' Addresses
Mingui Zhang Mingui Zhang
Huawei Technologies Huawei Technologies
No. 156 Beiqing Rd. Haidian District No. 156 Beiqing Rd. Haidian District
Beijing 100095 Beijing 100095
China China
Phone: +86-13810702575 Phone: +86-13810702575
Email: zhangmingui@huawei.com Email: zhangmingui@huawei.com
skipping to change at page 14, line 31 skipping to change at page 13, line 36
Email: zhangxudong@huawei.com Email: zhangxudong@huawei.com
Donald E. Eastlake, 3rd Donald E. Eastlake, 3rd
Huawei Technologies Huawei Technologies
155 Beaver Street 155 Beaver Street
Milford, MA 01757 Milford, MA 01757
United States United States
Phone: +1-508-333-2270 Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com Email: d3e3e3@gmail.com
Radia Perlman Radia Perlman
EMC EMC
2010 256th Avenue NE, #200 2010 256th Avenue NE, #200
Bellevue, WA 98007 Bellevue, WA 98007
United States United States
Email: radia@alum.mit.edu Email: radia@alum.mit.edu
Somnath Chatterjee Somnath Chatterjee
Cisco Systems Cisco Systems
SEZ Unit, Cessna Business Park SEZ Unit, Cessna Business Park
Outer Ring Road Outer Ring Road
Bangalore - 560087 Bangalore - 560087
India India
Email: somnath.chatterjee01@gmail.com Email: somnath.chatterjee01@gmail.com
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