Internet DRAFT - draft-ietf-bier-path-mtu-discovery
draft-ietf-bier-path-mtu-discovery
BIER Working Group G. Mirsky
Internet-Draft Ericsson
Intended status: Standards Track T. Przygienda
Expires: 8 May 2024 Juniper Networks
A. Dolganow
Nokia
5 November 2023
Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit
Replication (BIER) Layer
draft-ietf-bier-path-mtu-discovery-16
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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This Internet-Draft will expire on 8 May 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 2
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 2
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 smallest 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 path 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 path 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. [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.
1.1. Conventions used in this document
1.1.1. Terminology
This document uses terminology defined in [RFC8279]. Familiarity
with this specification and the terminology used is expected.
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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 transit node that
cannot forward a probe packet that is bigger than its link MTU sends
to the packet's 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.
Applying 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
repeatedly until the very last egress responds with a positive
acknowledgment. Consider the 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 unnecessarily 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.
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-----
--| D |
----- / -----
--| B |--
/ ----- \ -----
/ --| E |
----- / -----
| A |--- -----
----- \ --| F |
\ ----- / -----
--| C |--
----- \ -----
--| G |
-----
Figure 1: Multicast network
3. PMTUD Mechanism for BIER
A multicast distribution tree connects a BFIR with a set of BFERs via
procedures explained in [RFC8279]. The BFIR determines the MTU of
this multicast distribution tree by transmitting a series of probe
packets from BFIR to the set of BFERs. In the case of ECMP, BFIR MAY
test each path by variating the value in the Entropy field. The
critical step in the process of Path MTU discovery is the
notification of BFIR about the failure at an intermediate BFR to
forward the probe packet toward the subset of targeted downstream
BFERs. That is achieved by BFR responding with a partial (compared
to the one it received in the request) bitmask towards the
originating BFIR in error notification. That allows for the
retransmission of the next probe with a smaller MTU addressed only
toward a smaller set of BFERs downstream from the failed BFR instead
of all BFERs within the multicast distribution tree. In the scenario
discussed in Section 2, the second and all following (if needed)
probes will be sent only to node D because the smaller link MTU of
interface B-D. Since the MTU discovery of E, F, and G has been
completed already by the first probe, the second, and any of the
following probes will not be forwarded to these leaves.
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
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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 BIER-enabled
egress interfaces toward downstream BFRs that could be used to
reach B's downstream BFERs;
* Address Type MAY be set to any value defined in Section 3.3.4
[I-D.ietf-bier-ping].
* I flag MUST be cleared to direct the responding BFR not to include
the Incoming SI-BitString TLV in the BIER Echo Response.
* Downstream Interface Address field MUST be zeroed.
* List of Sub-TLVs MUST include the Egress Bitstring sub-TLV with
the list of all BFERs that cannot be reached because the egress
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 bitmask string (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')
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If a negative Echo Reply is received, the BFIR MUST wait for the
expiration of the Echo Request before transmitting the updated Echo
Request. 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 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, but not more
frequently than after the Echo Request timer expired, 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. Data TLV MAY
be added in BIER Echo Request or Echo Reply message as defined in
[I-D.ietf-bier-ping].
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 |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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]
Nainar, N. K., Pignataro, C., Chen, M., and G. Mirsky,
"BIER Ping and Trace", Work in Progress, Internet-Draft,
draft-ietf-bier-ping-12, 29 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
ping-12>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
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[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>.
[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>.
[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>.
7.2. Informative References
[I-D.ietf-bier-oam-requirements]
Mirsky, G., Nainar, N. K., 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-13, 10 August 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
oam-requirements-13>.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Tony Przygienda
Juniper Networks
Email: prz@juniper.net
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Andrew Dolganow
Nokia
Email: andrew.dolganow@nokia.com
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