draft-ietf-bier-path-mtu-discovery-03.txt   draft-ietf-bier-path-mtu-discovery-04.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: July 21, 2018 Juniper Networks Expires: December 21, 2018 Juniper Networks
A. Dolganow A. Dolganow
Nokia Nokia
January 17, 2018 June 19, 2018
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-03 draft-ietf-bier-path-mtu-discovery-04
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
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 21, 2018. This Internet-Draft will expire on December 21, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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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. Terminology . . . . . . . . . . . . . . . . . . . . . 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. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 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
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
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 a 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 along
the route that a set of packets will follow to reach the given set of the route that a set of packets will follow to reach the given set of
destinations. Such MTU determination along a specific path is 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 forwarding of multicast data
packets. A BIER domain consists of Bit-Forwarding Routers (BFRs) packets. A BIER domain consists of Bit-Forwarding Routers (BFRs)
that are uniquely identified by their respective BFR-ids. An ingress that are uniquely identified by their respective BFR-ids. An ingress
border router (acting as a Bit Forwarding Ingress Router (BFIR)) border router (acting as a Bit Forwarding Ingress Router (BFIR))
inserts a Forwarding Bit Mask (F-BM) into a packet. Each targeted inserts a Forwarding Bit Mask (F-BM) into a packet. Each targeted
egress node (referred to as a Bit Forwarding Egress Router (BFER)) is egress node (referred to as a Bit Forwarding Egress Router (BFER)) is
represented by Bit Mask Position (BMP) in the BMS. A transit or represented by Bit Mask Position (BMP) in the BMS. A transit or
intermediate BIER node, referred as BFR, forwards BIER encapsulated intermediate BIER node, referred to as BFR, forwards BIER
packets to BFERs, identified by respective BMPs, according to a Bit encapsulated packets to BFERs, identified by respective BMPs,
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. Terminology
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
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[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 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 acknowledgement. packet destination may respond with a positive acknowledgment. Thus,
Thus, possibly through a series of iterations, varying the size of possibly through a series of iterations, varying the size of the
the probe packet, the packet source discovers the PMTU of the probe packet, the packet source discovers the PMTU of the particular
particular path. path.
Thus applied such existing PMTUD solutions are inefficient for point- Thus applied such existing PMTUD solutions are inefficient for point-
to-multipoint paths constructed for multicast traffic. Probe packets to-multipoint paths constructed for multicast traffic. Probe packets
must be flooded through the whole set of multicast distribution paths must be flooded through the whole set of multicast distribution paths
over and over again until the very last egress responds with a over and over again until the very last egress responds with a
positive acknowledgement. Consider without loss of generality an positive acknowledgment. Consider without loss of generality an
example multicast network presented in Figure 1, where MTU on all example multicast network presented in Figure 1, where MTU on all
links but one (B,D) is the same. If MTU on link (B,D) is smaller links but one (B, D) is the same. If MTU on the link (B, D) is
than the MTU on the other links, using existing PMTUD mechanism smaller than the MTU on the other links, using existing PMTUD
probes will unnecessary flood to leaf nodes E, F, and G for the mechanism probes will unnecessary flood to leaf nodes E, F, and G for
second and consecutive times and positive responses will be generated the second and consecutive times and positive responses will be
and received by root A repeatedly. generated and received by root A repeatedly.
----- -----
--| D | --| D |
----- / ----- ----- / -----
--| B |-- --| B |--
/ ----- \ ----- / ----- \ -----
/ --| E | / --| E |
----- / ----- ----- / -----
| A |--- ----- | A |--- -----
----- \ --| F | ----- \ --| F |
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--| C |-- --| C |--
----- \ ----- ----- \ -----
--| 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 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 series of probe packets,
the MTU of that multicast distribution tree. The critical step is the MTU of that multicast distribution tree. In case of ECMP, BFIR
that in case of failure at an intermediate BFR to forward towards the MAY test each path by variating the value in Entropy field. The
subset of targeted downstream BFERs, the BFR responds with a partial critical step is that in case of failure at an intermediate BFR to
(compared to the one it received in the request) bitmask towards the forward towards the subset of targeted downstream BFERs, the BFR
originating BFIR in error notification. That allows for responds with a partial (compared to the one it received in the
retransmission of the next probe with smaller MTU address only request) bitmask towards the originating BFIR in error notification.
towards the failed downstream BFERs instead of all BFERs addressed in That allows for retransmission of the next probe with smaller MTU
the previous probe. In the scenario discussed in Section 2 the address only towards the failed downstream BFERs instead of all BFERs
second and all following (if needed) probes will be sent only to the addressed in the previous probe. In the scenario discussed in
node D since MTU discovery of E, F, and G has been completed already Section 2 the second and all following (if needed) probes will be
by the first probe successfully. sent only to the node D since MTU discovery of E, F, and G has been
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.
Consider the network displayed in Figure 1 to be presentation of a Consider the network displayed in Figure 1 to be a presentation of a
BIER domain and all nodes to be BFRs. To discover MTU over BIER BIER domain and all nodes to be BFRs. To discover MTU over BIER
domain to BFERs D, F, E, and G BFIR A will use BIER Ping with Data domain to BFERs D, F, E, and G BFIR A will use BIER Ping with Data
TLV, defined in Section 3.1. Size of the first probe set to M_max TLV, defined in Section 3.1. Size of the first probe set to M_max
determined as minimal MTU value of BFIR's links to BIER domain. As determined as minimal MTU value of BFIR's links to BIER domain. As
has been assumed in Section 2, MTUs of all links but link (B,D) are has been assumed in Section 2, MTUs of all links but the link (B, D)
the same. Thus BFERs E. F, and G would receive BIER Echo Request are the same. Thus BFERs E, F, and G would receive BIER Echo Request
and will send their respective replies to BFIR A. BFR B may pass the and will send their respective replies to BFIR A. BFR B may pass the
packet which is too large to forward over egress link (B, D) to the packet which is too large to forward over egress link (B, D) to the
appropriate network layer for error processing where it would be appropriate network layer for error processing where it would be
recognized as a BIER Echo Request packet. BFR B MUST send BIER Echo recognized as a BIER Echo Request packet. BFR B MUST send BIER Echo
Reply to BFIR A and MUST include Downstream Mapping TLV, defined in Reply to BFIR A and MUST include Downstream Mapping TLV, defined in
[I-D.ietf-bier-ping] setting its fields in the following fashion: [I-D.ietf-bier-ping] setting its fields in the following fashion:
o MTU SHOULD be set to the minimal MTU value among all egress BIER o MTU SHOULD be set to the minimal MTU value among all egress BIER
links, logical links between this and downstream BFRs, that could links, logical links between this and downstream BFRs, that could
be used to reach B's downstream BFERs; be used to reach B's downstream BFERs;
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o I flag MUST be cleared; o I flag MUST be cleared;
o Downstream Interface Address field (4 octets) MUST be zeroed and o Downstream Interface Address field (4 octets) MUST be zeroed and
MUST include in the Egress Bitstring sub-TLV the list of all BFERs MUST include in the Egress Bitstring sub-TLV the list of all BFERs
that cannot be reached because the attempted MTU turned out to be that cannot be reached because the attempted MTU turned out to be
too small. too small.
The BFIR will receive either of the two types of packets: The BFIR will receive either of the two types of packets:
o a positive Echo Reply from one of BFERs to which the probe has o a positive Echo Reply from one of BFERs to which the probe has
been sent. In this case the bit corresponding to the BFER MUST be been sent. In this case, the bit corresponding to the BFER MUST
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 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, if upon expiration of the Echo
Request timer BFIR didn't receive any Echo Reply, then BFIR MAY Request timer BFIR didn't receive any Echo Reply, then BFIR MAY
continue to retransmit the probe using the initial size and MAY apply continue to retransmit the probe using the initial size and MAY apply
probe delay retransmission procedures. The algorithm used to delay probe delay retransmission procedures. The algorithm used to delay
retransmission procedures on BFIR is outside the scope of this retransmission procedures on BFIR is outside the scope of this
specification. The BFIR sends probes using BMS and locally defined specification. The BFIR sends probes using BMS and locally defined
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| TBA1 | Data | This document | | TBA1 | Data | This document |
+-------+-------------+---------------+ +-------+-------------+---------------+
Table 1: Data TLV Type Table 1: Data TLV Type
5. Security Considerations 5. Security Considerations
Routers that support PMTUD based on this document are subject to the Routers that support PMTUD based on this document are subject to the
same security considerations as defined in [I-D.ietf-bier-ping] same security considerations as defined in [I-D.ietf-bier-ping]
6. Acknowledgement 6. Acknowledgment
Authors greatly appreciate thorough review and the most detailed Authors greatly appreciate thorough review and the most detailed
comments by Eric Gray. comments by Eric Gray.
7. References 7. References
7.1. Normative References 7.1. Normative References
[I-D.ietf-bier-ping] [I-D.ietf-bier-ping]
Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M., Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M.,
and G. Mirsky, "BIER Ping and Trace", draft-ietf-bier- and G. Mirsky, "BIER Ping and Trace", draft-ietf-bier-
ping-02 (work in progress), July 2017. ping-03 (work in progress), January 2018.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990, DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>. <https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 8, line 17 skipping to change at page 8, line 17
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., Nordmark, E., Pignataro, C., Kumar, N., Mirsky, G., Nordmark, E., Pignataro, C., Kumar, N.,
Aldrin, S., Zheng, L., Chen, M., Akiya, N., and S. Aldrin, S., Zheng, L., Chen, M., Akiya, N., and S.
Pallagatti, "Operations, Administration and Maintenance Pallagatti, "Operations, Administration and Maintenance
(OAM) Requirements for Bit Index Explicit Replication (OAM) Requirements for Bit Index Explicit Replication
(BIER) Layer", draft-ietf-bier-oam-requirements-04 (work (BIER) Layer", draft-ietf-bier-oam-requirements-05 (work
in progress), July 2017. in progress), January 2018.
[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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