Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit Replication (BIER) Layer
draft-ietf-bier-path-mtu-discovery-11
BIER Working Group G. Mirsky
Internet-Draft Ericsson
Intended status: Standards Track T. Przygienda
Expires: 7 April 2022 Juniper Networks
A. Dolganow
Individual contributor
4 October 2021
Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit
Replication (BIER) Layer
draft-ietf-bier-path-mtu-discovery-11
Abstract
This document describes Path Maximum Transmission Unit Discovery
(PMTUD) in Bit Indexed Explicit Replication (BIER) layer.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. PMTUD Mechanism for BIER . . . . . . . . . . . . . . . . . . 4
3.1. Data TLV for BIER Ping . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
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
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)
for any forwarding device along the routed path to the IP destination
for these packets. Such "least MTU" is known as Path MTU (PMTU).
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
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 route that a set of packets will follow to reach the given
set of destinations. Such MTU determination along a specific path is
referred to as path MTU discovery (PMTUD).
[RFC8279] introduces and explains Bit Index Explicit Replication
(BIER) architecture and how it supports the forwarding of multicast
data packets. A BIER domain consists of Bit-Forwarding Routers
(BFRs) that are uniquely identified by their respective BFR-ids. An
ingress border router (acting as a Bit Forwarding Ingress Router
(BFIR)) inserts a Forwarding Bit Mask (F-BM) into a packet. Each
targeted egress node (referred to as a Bit Forwarding Egress Router
(BFER)) is represented by Bit Mask Position (BMP) in the BMS. A
transit or intermediate BIER node, referred to as BFR, forwards BIER
encapsulated packets to BFERs, identified by respective BMPs,
according to a Bit Index Forwarding Table (BIFT).
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1.1. Conventions used in this document
1.1.1. Acronyms
BFR: Bit-Forwarding Router
BFER: Bit-Forwarding Egress Router
BFIR: Bit-Forwarding Ingress Router
BIER: Bit Index Explicit Replication
BIFT: Bit Index Forwarding Tree
F-BM: Forwarding Bit Mask
MTU: Maximum Transmission Unit
OAM: Operations, Administration and Maintenance
PMTUD: Path MTU Discovery
1.1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Problem Statement
[I-D.ietf-bier-oam-requirements] sets forth the requirement to define
PMTUD protocol for BIER domain. This document describes the
extension to [I-D.ietf-bier-ping] for use in the BIER PMTUD solution.
Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are
primarily targeted to work on point-to-point, i.e. unicast paths.
These mechanisms use packet fragmentation control by disabling
fragmentation of the probe packet. As a result, a transient node
that cannot forward a probe packet that is bigger than its link MTU
sends to the packet source an error notification, otherwise the
packet destination may respond with a positive acknowledgment. Thus,
possibly through a series of iterations, varying the size of the
probe packet, the packet source discovers the PMTU of the particular
path.
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Thus applied such existing PMTUD solutions are inefficient for point-
to-multipoint paths constructed for multicast traffic. Probe packets
must be flooded through the whole set of multicast distribution paths
over and over again until the very last egress responds with a
positive acknowledgment. Consider without loss of generality an
example multicast network presented in Figure 1, where MTU on all
links but one (B, D) is the same. If MTU on the link (B, D) is
smaller than the MTU on the other links, using existing PMTUD
mechanism probes will unnecessary flood to leaf nodes E, F, and G for
the second and consecutive times and positive responses will be
generated and received by root A repeatedly.
-----
--| D |
----- / -----
--| B |--
/ ----- \ -----
/ --| E |
----- / -----
| A |--- -----
----- \ --| F |
\ ----- / -----
--| C |--
----- \ -----
--| G |
-----
Figure 1: Multicast network
3. PMTUD Mechanism for BIER
A BFIR selects a set of BFERs for the specific multicast
distribution. Such a BFIR determines, by explicitly controlling a
subset of targeted BFERs and transmitting a series of probe packets,
the MTU of that multicast distribution tree. In the case of ECMP,
BFIR MAY test each path by variating the value in the Entropy field.
The critical step is that in case of failure at an intermediate BFR
to forward towards the subset of targeted downstream BFERs, the BFR
responds with a partial (compared to the one it received in the
request) bitmask towards the originating BFIR in error notification.
That allows for retransmission of the next probe with a smaller MTU
address only towards the failed downstream BFERs instead of all BFERs
addressed in the previous probe. In the scenario discussed in
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
completed already by the first probe successfully.
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[I-D.ietf-bier-ping] introduced BIER Ping as a transport-independent
OAM mechanism to detect and localize failures in the BIER data plane.
This document specifies how BIER Ping can be used to perform
efficient PMTUD in the BIER domain.
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
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
determined as minimal MTU value of BFIR's links to BIER domain. As
has been assumed in Section 2, MTUs of all links but the link (B, D)
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
packet which is too large to forward over egress link (B, D) to the
appropriate network layer for error processing where it would be
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
[I-D.ietf-bier-ping] setting its fields in the following fashion:
* MTU SHOULD be set to the minimal MTU value among all egress BIER
links, logical links between this and downstream BFRs, that could
be used to reach B's downstream BFERs;
* Address Type MUST be set to 0 [Ed.note: we need to define 0 as
valid value for the Address Type field with the specific semantics
to "Ignore" it.]
* I flag MUST be cleared;
* Downstream Interface Address field (4 octets) MUST be zeroed and
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
too small.
The BFIR will receive either of the two types of packets:
* 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 cleared from the BMS;
* a negative Echo Reply with bit string listing unreached BFERs and
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')
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
are scenarios when an implementation of the PMTUD would not decrease
the size of the probe. For example, suppose upon expiration of the
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Echo Request timer BFIR didn't receive any Echo Reply. In that case,
BFIR MAY continue to retransmit the probe using the initial size and
MAY apply probe delay retransmission procedures. The algorithm used
to delay retransmission procedures on BFIR is outside the scope of
this specification. The BFIR sends probes using BMS and locally
defined retransmission procedures until either the bit string is
clear, i.e., contains no set bits, or until the BFIR retransmission
procedure terminates and PMTU discovery is declared unsuccessful. In
the case of convergence of the procedure, the size of the last probe
indicates the PMTU size that can be used for all BFERs in the initial
BMS without incurring fragmentation.
Thus we conclude that in order to comply with the requirement in
[I-D.ietf-bier-oam-requirements]:
* a BFR SHOULD support PMTUD;
* 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
reach BFERs;
* a BFIR MUST have a locally defined PMTUD probe retransmission
procedure.
3.1. Data TLV for BIER Ping
There needs to be a control for probe size in order to support the
BIER PMTUD. Data TLV format is presented in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBA1) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Data TLV format
* Type: indicates Data TLV, to be allocated by IANA Section 4.
* Length: the length of the Data field in octets.
* Data: n octets (n = Length) of arbitrary data. The receiver
SHOULD ignore it.
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4. IANA Considerations
IANA is requested to assign a new Type value for Data TLV Type from
its registry of TLV and sub-TLV Types of BIER Ping as follows:
+=======+=============+===============+
| Value | Description | Reference |
+=======+=============+===============+
| TBA1 | Data | This document |
+-------+-------------+---------------+
Table 1: Data TLV Type
5. Security Considerations
Routers that support PMTUD based on this document are subject to the
same security considerations as defined in [I-D.ietf-bier-ping]
6. Acknowledgment
Authors greatly appreciate thorough review and the most detailed
comments by Eric Gray.
7. References
7.1. Normative References
[I-D.ietf-bier-ping]
Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M.,
and G. Mirsky, "BIER Ping and Trace", Work in Progress,
Internet-Draft, draft-ietf-bier-ping-07, 11 May 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
ping-07>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
7.2. Informative References
[I-D.ietf-bier-oam-requirements]
Mirsky, G., Kumar, N., Chen, M., and S. Pallagatti,
"Operations, Administration and Maintenance (OAM)
Requirements for Bit Index Explicit Replication (BIER)
Layer", Work in Progress, Internet-Draft, draft-ietf-bier-
oam-requirements-11, 15 November 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
oam-requirements-11>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Tony Przygienda
Juniper Networks
Email: prz@juniper.net
Andrew Dolganow
Individual contributor
Email: adolgano@gmail.com
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