draft-ietf-bier-path-mtu-discovery-08.txt   draft-ietf-bier-path-mtu-discovery-09.txt 
BIER Working Group G. Mirsky BIER Working Group G. Mirsky
Internet-Draft ZTE Corp. Internet-Draft ZTE Corp.
Intended status: Standards Track T. Przygienda Intended status: Standards Track T. Przygienda
Expires: November 22, 2020 Juniper Networks Expires: May 23, 2021 Juniper Networks
A. Dolganow A. Dolganow
Individual contributor Individual contributor
May 21, 2020 November 19, 2020
Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit
Replication (BIER) Layer Replication (BIER) Layer
draft-ietf-bier-path-mtu-discovery-08 draft-ietf-bier-path-mtu-discovery-09
Abstract Abstract
This document describes Path Maximum Transmission Unit Discovery This document describes Path Maximum Transmission Unit Discovery
(PMTUD) in Bit Indexed Explicit Replication (BIER) layer. (PMTUD) in Bit Indexed Explicit Replication (BIER) layer.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 22, 2020. This Internet-Draft will expire on May 23, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3 1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3 1.1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3 1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. PMTUD Mechanism for BIER . . . . . . . . . . . . . . . . . . 4 3. PMTUD Mechanism for BIER . . . . . . . . . . . . . . . . . . 4
3.1. Data TLV for BIER Ping . . . . . . . . . . . . . . . . . 6 3.1. Data TLV for BIER Ping . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 7 6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . 8
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In packet switched networks, when a host seeks to transmit data to a In packet switched networks, when a host seeks to transmit data to a
target destination, the data is transmitted as a set of packets. In target destination, the data is transmitted as a set of packets. In
many cases, it is more efficient to use the largest size packets that many cases, it is more efficient to use the largest size packets that
are less than or equal to the least Maximum Transmission Unit (MTU) are less than or equal to the least Maximum Transmission Unit (MTU)
for any forwarding device along the routed path to the IP destination for any forwarding device along the routed path to the IP destination
for these packets. Such "least MTU" is known as Path MTU (PMTU). for these packets. Such "least MTU" is known as Path MTU (PMTU).
Fragmentation or packet drop, silent or not, may occur on hops along Fragmentation or packet drop, silent or not, may occur on hops along
the route where an MTU is smaller than the size of the datagram. To the route where an MTU is smaller than the size of the datagram. To
avoid any of the listed above behaviors, the packet source must find avoid any of the listed above behaviors, the packet source must find
the value of the least MTU, i.e. PMTU, that will be encountered along the value of the least MTU, i.e., PMTU, that will be encountered
the route that a set of packets will follow to reach the given set of along the route that a set of packets will follow to reach the given
destinations. Such MTU determination along a specific path is set of destinations. Such MTU determination along a specific path is
referred to as path MTU discovery (PMTUD). referred to as path MTU discovery (PMTUD).
[RFC8279] introduces and explains Bit Index Explicit Replication [RFC8279] introduces and explains Bit Index Explicit Replication
(BIER) architecture and how it supports forwarding of multicast data (BIER) architecture and how it supports the forwarding of multicast
packets. A BIER domain consists of Bit-Forwarding Routers (BFRs) data packets. A BIER domain consists of Bit-Forwarding Routers
that are uniquely identified by their respective BFR-ids. An ingress (BFRs) that are uniquely identified by their respective BFR-ids. An
border router (acting as a Bit Forwarding Ingress Router (BFIR)) ingress border router (acting as a Bit Forwarding Ingress Router
inserts a Forwarding Bit Mask (F-BM) into a packet. Each targeted (BFIR)) inserts a Forwarding Bit Mask (F-BM) into a packet. Each
egress node (referred to as a Bit Forwarding Egress Router (BFER)) is targeted egress node (referred to as a Bit Forwarding Egress Router
represented by Bit Mask Position (BMP) in the BMS. A transit or (BFER)) is represented by Bit Mask Position (BMP) in the BMS. A
intermediate BIER node, referred to as BFR, forwards BIER transit or intermediate BIER node, referred to as BFR, forwards BIER
encapsulated packets to BFERs, identified by respective BMPs, encapsulated packets to BFERs, identified by respective BMPs,
according to a Bit Index Forwarding Table (BIFT). according to a Bit Index Forwarding Table (BIFT).
1.1. Conventions used in this document 1.1. Conventions used in this document
1.1.1. Terminology 1.1.1. Acronyms
BFR: Bit-Forwarding Router BFR: Bit-Forwarding Router
BFER: Bit-Forwarding Egress Router BFER: Bit-Forwarding Egress Router
BFIR: Bit-Forwarding Ingress Router BFIR: Bit-Forwarding Ingress Router
BIER: Bit Index Explicit Replication BIER: Bit Index Explicit Replication
BIFT: Bit Index Forwarding Tree BIFT: Bit Index Forwarding Tree
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Problem Statement 2. Problem Statement
[I-D.ietf-bier-oam-requirements] sets forth the requirement to define [I-D.ietf-bier-oam-requirements] sets forth the requirement to define
PMTUD protocol for BIER domain. This document describes the PMTUD protocol for BIER domain. This document describes the
extension to [I-D.ietf-bier-ping] for use in BIER PMTUD solution. extension to [I-D.ietf-bier-ping] for use in the BIER PMTUD solution.
Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are
primarily targeted to work on point-to-point, i.e. unicast paths. primarily targeted to work on point-to-point, i.e. unicast paths.
These mechanisms use packet fragmentation control by disabling These mechanisms use packet fragmentation control by disabling
fragmentation of the probe packet. As a result, a transient node fragmentation of the probe packet. As a result, a transient node
that cannot forward a probe packet that is bigger than its link MTU that cannot forward a probe packet that is bigger than its link MTU
sends to the packet source an error notification, otherwise the sends to the packet source an error notification, otherwise the
packet destination may respond with a positive acknowledgment. Thus, packet destination may respond with a positive acknowledgment. Thus,
possibly through a series of iterations, varying the size of the possibly through a series of iterations, varying the size of the
probe packet, the packet source discovers the PMTU of the particular probe packet, the packet source discovers the PMTU of the particular
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----- \ ----- ----- \ -----
--| G | --| G |
----- -----
Figure 1: Multicast network Figure 1: Multicast network
3. PMTUD Mechanism for BIER 3. PMTUD Mechanism for BIER
A BFIR selects a set of BFERs for the specific multicast A BFIR selects a set of BFERs for the specific multicast
distribution. Such a BFIR determines, by explicitly controlling a distribution. Such a BFIR determines, by explicitly controlling a
subset of targeted BFERs and transmitting series of probe packets, subset of targeted BFERs and transmitting a series of probe packets,
the MTU of that multicast distribution tree. In case of ECMP, BFIR the MTU of that multicast distribution tree. In the case of ECMP,
MAY test each path by variating the value in Entropy field. The BFIR MAY test each path by variating the value in the Entropy field.
critical step is that in case of failure at an intermediate BFR to The critical step is that in case of failure at an intermediate BFR
forward towards the subset of targeted downstream BFERs, the BFR to forward towards the subset of targeted downstream BFERs, the BFR
responds with a partial (compared to the one it received in the responds with a partial (compared to the one it received in the
request) bitmask towards the originating BFIR in error notification. request) bitmask towards the originating BFIR in error notification.
That allows for retransmission of the next probe with smaller MTU That allows for retransmission of the next probe with a smaller MTU
address only towards the failed downstream BFERs instead of all BFERs address only towards the failed downstream BFERs instead of all BFERs
addressed in the previous probe. In the scenario discussed in addressed in the previous probe. In the scenario discussed in
Section 2 the second and all following (if needed) probes will be Section 2 the second and all following (if needed) probes will be
sent only to the node D since MTU discovery of E, F, and G has been sent only to the node D since MTU discovery of E, F, and G has been
completed already by the first probe successfully. completed already by the first probe successfully.
[I-D.ietf-bier-ping] introduced BIER Ping as a transport-independent [I-D.ietf-bier-ping] introduced BIER Ping as a transport-independent
OAM mechanism to detect and localize failures in the BIER data plane. OAM mechanism to detect and localize failures in the BIER data plane.
This document specifies how BIER Ping can be used to perform This document specifies how BIER Ping can be used to perform
efficient PMTUD in the BIER domain. efficient PMTUD in the BIER domain.
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been sent. In this case, the bit corresponding to the BFER MUST been sent. In this case, the bit corresponding to the BFER MUST
be cleared from the BMS; be cleared from the BMS;
o a negative Echo Reply with bit string listing unreached BFERs and o a negative Echo Reply with bit string listing unreached BFERs and
recommended MTU value MTU'. The BFIR MUST add the bit string to recommended MTU value MTU'. The BFIR MUST add the bit string to
its BMS and set the size of the next probe as min(MTU, MTU') its BMS and set the size of the next probe as min(MTU, MTU')
If upon expiration of the Echo Request timer BFIR didn't receive any If upon expiration of the Echo Request timer BFIR didn't receive any
Echo Replies, then the size of the probe SHOULD be decreased. There Echo Replies, then the size of the probe SHOULD be decreased. There
are scenarios when an implementation of the PMTUD would not decrease are scenarios when an implementation of the PMTUD would not decrease
the size of the probe. For example, if upon expiration of the Echo the size of the probe. For example, suppose upon expiration of the
Request timer BFIR didn't receive any Echo Reply, then BFIR MAY Echo Request timer BFIR didn't receive any Echo Reply. In that case,
continue to retransmit the probe using the initial size and MAY apply BFIR MAY continue to retransmit the probe using the initial size and
probe delay retransmission procedures. The algorithm used to delay MAY apply probe delay retransmission procedures. The algorithm used
retransmission procedures on BFIR is outside the scope of this to delay retransmission procedures on BFIR is outside the scope of
specification. The BFIR sends probes using BMS and locally defined this specification. The BFIR sends probes using BMS and locally
retransmission procedures until either the bit string is clear, i.e. defined retransmission procedures until either the bit string is
contains no set bits, or until the BFIR retransmission procedure clear, i.e., contains no set bits, or until the BFIR retransmission
terminates and PMTU discovery is declared unsuccessful. In case of procedure terminates and PMTU discovery is declared unsuccessful. In
convergence of the procedure, the size of the last probe indicates the case of convergence of the procedure, the size of the last probe
the PMTU size that can be used for all BFERs in the initial BMS indicates the PMTU size that can be used for all BFERs in the initial
without incurring fragmentation. BMS without incurring fragmentation.
Thus we conclude that in order to comply with the requirement in Thus we conclude that in order to comply with the requirement in
[I-D.ietf-bier-oam-requirements]: [I-D.ietf-bier-oam-requirements]:
o a BFR SHOULD support PMTUD; o a BFR SHOULD support PMTUD;
o a BFR MAY use defined per BIER sub-domain MTU value as initial MTU o a BFR MAY use defined per BIER sub-domain MTU value as initial MTU
value for discovery or use it as MTU for this BIER sub-domain to value for discovery or use it as MTU for this BIER sub-domain to
reach BFERs; reach BFERs;
o a BFIR MUST have a locally defined of PMTUD probe retransmission o a BFIR MUST have a locally defined PMTUD probe retransmission
procedure. procedure.
3.1. Data TLV for BIER Ping 3.1. Data TLV for BIER Ping
There needs to be a control for probe size in order to support the There needs to be a control for probe size in order to support the
BIER PMTUD. Data TLV format is presented in Figure 2. BIER PMTUD. Data TLV format is presented in Figure 2.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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o Type: indicates Data TLV, to be allocated by IANA Section 4. o Type: indicates Data TLV, to be allocated by IANA Section 4.
o Length: the length of the Data field in octets. o Length: the length of the Data field in octets.
o Data: n octets (n = Length) of arbitrary data. The receiver o Data: n octets (n = Length) of arbitrary data. The receiver
SHOULD ignore it. SHOULD ignore it.
4. IANA Considerations 4. IANA Considerations
IANA is requested to assign new Type value for Data TLV Type from its IANA is requested to assign a new Type value for Data TLV Type from
registry of TLV and sub-TLV Types of BIER Ping as follows: its registry of TLV and sub-TLV Types of BIER Ping as follows:
+-------+-------------+---------------+ +-------+-------------+---------------+
| Value | Description | Reference | | Value | Description | Reference |
+-------+-------------+---------------+ +-------+-------------+---------------+
| TBA1 | Data | This document | | TBA1 | Data | This document |
+-------+-------------+---------------+ +-------+-------------+---------------+
Table 1: Data TLV Type Table 1: Data TLV Type
5. Security Considerations 5. Security Considerations
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May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201, "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017, DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>. <https://www.rfc-editor.org/info/rfc8201>.
7.2. Informative References 7.2. Informative References
[I-D.ietf-bier-oam-requirements] [I-D.ietf-bier-oam-requirements]
Mirsky, G., Nainar, N., Chen, M., and J. Networks, Mirsky, G., Nainar, N., Chen, M., and S. Pallagatti,
"Operations, Administration and Maintenance (OAM) "Operations, Administration and Maintenance (OAM)
Requirements for Bit Index Explicit Replication (BIER) Requirements for Bit Index Explicit Replication (BIER)
Layer", draft-ietf-bier-oam-requirements-10 (work in Layer", draft-ietf-bier-oam-requirements-11 (work in
progress), May 2020. progress), November 2020.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279, Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017, DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>. <https://www.rfc-editor.org/info/rfc8279>.
Authors' Addresses Authors' Addresses
Greg Mirsky Greg Mirsky
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