Internet DRAFT - draft-bonica-intarea-lossless-pmtud
draft-bonica-intarea-lossless-pmtud
INTAREA WG R. Bonica
Internet-Draft M. Nayak
Intended status: Experimental Juniper Networks
Expires: May 3, 2020 B. Newton
H. Alpan
R. Rosborough
M. President
Harvey Mudd College
October 31, 2019
Lossless Path MTU Discovery (PMTUD)
draft-bonica-intarea-lossless-pmtud-01
Abstract
This document describes alternative IPv4 PMTUD procedures that do not
prevent IP fragmentation and do no rely on the network's ability to
deliver ICMP Destination Unreachable messages to the source node.
This document also defines a new ICMP message. IPv4 nodes emit this
new message when they reassemble a fragmented packet.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 3, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. The ICMP Packet Reassembly Message . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
For reasons described in [RFC1191], IPv4 source nodes estimate the
Path MTU (PMTU) between themselves and destination nodes. An
extremely conservative source node estimates the PMTU for each path
to be equal to the IPv4 Minimum Link MTU (See Note 1). While such
conservative estimates are guaranteed to be less than or equal to the
actual PMTU, they are likely to be much less than the actual PMTU.
This may adversely affect upper-layer protocol performance.
By executing PMTU Discovery (PMTUD) [RFC1191] procedures, IPv4 source
nodes can maintain less conservative PMTU estimates. In PMTUD, the
source node produces an initial PMTU estimate. This initial estimate
is equal to the MTU of the first link along the path to the
destination node. It can be greater than the actual PMTU.
Having produced an initial PMTU estimate, the source node sends non-
fragmentable packets to the destination node (see NOTE 2). If one of
these packets is larger than the actual PMTU, a downstream router
will not be able to forward the packet through the next link along
the path. Therefore, the downstream router drops the packet and
sends an Internet Control Message Protocol (ICMP) [RFC0792]
Destination Unreachable message to the source node. The Code field
in the ICMP message is set to (4) "fragmentation needed". The ICMP
message also indicates the MTU of the link through which the packet
could not be forwarded. The source node uses this information to
refine its PMTU estimate.
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PMTUD produces a running estimate of the PMTU between a source node
and a destination node. Because PMTU is dynamic, the PMTU estimate
can be larger than the actual PMTU. In order to detect PMTU
increases, PMTUD occasionally resets the PMTU estimate to its initial
value and repeats the procedure described above.
Ideally, PMTUD operates as described above. However, PMTUD relies on
the network's ability to deliver ICMP Destination Unreachable
messages to the source node. If the network cannot deliver ICMP
Destination Unreachable messages to the source node, PMTUD fails and
connectivity may be lost.
This document describes alternative PMTUD procedures that do no rely
on the network's ability to deliver ICMP Destination Unreachable
messages to the source node. In these procedures, the source node
produces an initial PMTU estimate. This initial estimate is equal to
the MTU of the first link along the path to the destination node. It
can be greater than the actual PMTU.
Having produced an initial PMTU estimate, the source node sends
fragmentable packets to the destination node. If one of these
packets is larger than the actual PMTU, a downstream router will not
be able to forward the packet, in one piece, through the next link
along the path. Therefore, the downstream router fragments the
packet and forwards each fragment to the destination node. The
destination node reassembles the packet and sends an informational
ICMP message to the source node. The informational message indicates
that a packet has been reassembled. It also indicates the size of
the largest fragment received and contains as much of the original
packet as possible without causing the ICMP message to exceed its
maximum allowable size (i.e., 576 bytes).
The source node should use information contained by the message to
refine its PMTU estimate. Having refined its PMTU estimate, the
source node should refrain from sending packet long enough to require
fragmentation. However, the message may be lost by the network or
ignored by the source node. In this case, the source node may
continue to send packets that require fragmentation and reassembly.
In order to detect PMTU increases, the above-mentioned PMTUD
procedures occasionally resets the PMTU estimate to its initial value
and repeat the procedure described above.
This document defines the new ICMP message, mentioned above. The
PMTUD procedures described herein are applicable to IPv4 only,
because [RFC8200] does not allow fragmentation by transit nodes.
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This document does not update [RFC1191]. A source node can executed
the PMTUD procedures described herein in addition to [RFC1191]
procedures or instead of [RFC1191] procedures.
NOTE 1: In IPv4, every host must be capable of receiving a packet
whose length is equal to 576 bytes. However, the IPv4 minimum link
MTU is not 576. Section 3.2 of [RFC0791] states that the IPv4
minimum link MTU is 68 bytes. But for practical purposes, many
network operators consider the IPv4 minimum link MTU to be 576 bytes.
So, for the purposes of this document, we assume that the IPv4
minimum link MTU is 576 bytes.
NOTE 2: The DF-bit in the IPv4 header distinguishes fragmentable IPv4
packets from non-fragmentable IPv4 packets. If the DF-bit is equal
to 0, the packet is fragmentable. If the DF-bit equals 1, the packet
is not fragmentable.
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.
3. The ICMP Packet Reassembly Message
IPv4 nodes can emit an ICMP Packet Reassembly message when they
reassemble a packet. Figure 1 depicts the ICMP Packet Reassembly
message.
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Length | Largest Fragment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original Datagram |
| |
| // |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The ICMP Packet Reassembly Message
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o Type (8 bits) - Packet Reassembly. Value 253 (Experiment 1)
o Code (8-bits) - No Error (0), or Reassembly Error (1).
o Checksum (16 bits) - See [RFC0792].
o Unused (8 bits) - SHOULD be set to zero by sender. MUST be
ignored by receiver.
o Length (8 bits) - Length of the padded "original datagram" field,
measured in 32-bit words.
o Largest Fragment (16-bits) - Size of the largest fragment
received, measured in bytes,
o Original Datagram (variable length) - As much of the original
packet as possible, without exceeding the maximum size of an ICMP
message (576 bytes). Must be padded to 32-bit boundary. If Code
equals Reassembly Error, this field contains the first fragment.
As per [RFC1812], all ICMP messages, including the ICMP Packet
Reassembly message, SHOULD be rate limited.
The Code field is included for informational purposes only. The
receiving node SHOULD refine its PMTU estimate, regardless of the
value contained by the code field.
4. Security Considerations
Security considerations for the procedures described herein are
identical to those described for PMTUD. See Section 8 of [RFC1191].
[RFC5927]offers mitigations.
5. IANA Considerations
This document requires no IANA actions.
6. Acknowledgements
Thanks to TBD for their careful review of this document.
7. References
7.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
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[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[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>.
[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
7.2. Informative References
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>.
Authors' Addresses
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
USA
Email: rbonica@juniper.net
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Manoj Nayak
Juniper Networks
Bangalore, KA 560103
India
Email: manojnayak@juniper.net
Bradley Newton
Harvey Mudd College
340 Foothill Blvd.
Claremont, California 91711
USA
Email: bnewton@hmc.edu
Hakan Alpan
Harvey Mudd College
340 Foothill Blvd.
Claremont, California 91711
USA
Email: halpan@hnc.edu
Radon Rosborough
Harvey Mudd College
340 Foothill Blvd.
Claremont, California 91711
USA
Email: rrosborough@hmc.edu
Miles President
Harvey Mudd College
340 Foothill Blvd.
Claremont, California 91711
USA
Email: mpresident@hmc.edu
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