IP Parcels
draft-templin-intarea-parcels-12
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| Author | Fred Templin | ||
| Last updated | 2022-07-11 | ||
| Replaced by | draft-templin-intarea-parcels2 | ||
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draft-templin-intarea-parcels-12
Network Working Group F. L. Templin, Ed.
Internet-Draft Boeing Research & Technology
Updates: RFC2675 (if approved) 11 July 2022
Intended status: Standards Track
Expires: 12 January 2023
IP Parcels
draft-templin-intarea-parcels-12
Abstract
IP packets (both IPv4 and IPv6) are understood to contain a unit of
data which becomes the retransmission unit in case of loss. Upper
layer protocols including the Transmission Control Protocol (TCP) and
transports over the User Datagram Protocol (UDP) prepare data units
known as "segments", with traditional arrangements including a single
segment per IP packet. This document presents a new construct known
as the "IP Parcel" which permits a single packet to carry multiple
segments, essentially creating a "packet-of-packets". IP parcels
provide an essential building block for accommodating larger Maximum
Transmission Units (MTUs) in the Internet as discussed in this
document.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 January 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background and Motivation . . . . . . . . . . . . . . . . . . 4
4. IP Parcel Formation . . . . . . . . . . . . . . . . . . . . . 6
5. UDP Parcels . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. TCP Parcels . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Transmission of IP Parcels . . . . . . . . . . . . . . . . . 9
8. Parcel Path Qualification . . . . . . . . . . . . . . . . . . 12
9. Integrity . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. RFC2675 Updates . . . . . . . . . . . . . . . . . . . . . . . 16
11. IPv4 Jumbograms . . . . . . . . . . . . . . . . . . . . . . . 17
12. Implementation Status . . . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
14. Security Considerations . . . . . . . . . . . . . . . . . . . 17
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
16.1. Normative References . . . . . . . . . . . . . . . . . . 18
16.2. Informative References . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
IP packets (both IPv4 [RFC0791] and IPv6 [RFC8200]) are understood to
contain a unit of data which becomes the retransmission unit in case
of loss. Upper layer protocols including the Transmission Control
Protocol (TCP) [RFC0793] and transports over the User Datagram
Protocol (UDP) [RFC0768] (including QUIC [RFC9000], LTP [RFC5326] and
others) prepare data units known as "segments", with traditional
arrangements including a single segment per IP packet. This document
presents a new construct known as the "IP Parcel" which permits a
single packet to carry multiple segments. This essentially creates a
"packet-of-packets" with the IP layer and full TCP/UDP headers
appearing only once but with possibly multiple upper layer protocol
segments included.
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Parcels are formed when an upper layer protocol entity identified by
the "5-tuple" (source address, destination address, source port,
destination port, protocol number) prepares a data buffer with the
concatenation of up to 64 properly-formed segments that can be broken
out into smaller parcels using a copy of the IP and TCP/UDP header.
All segments except the final segment must be equal in size and no
larger than 65535 octets (minus headers), while the final segment
must not be larger than the others but may be smaller. The upper
layer protocol entity then delivers the buffer and non-final segment
size to the IP layer, which appends the necessary IP header plus
extensions to identify this as a parcel and not an ordinary packet.
Parcels can be forwarded over consecutive parcel-capable IP links in
the path until arriving at an ingress middlebox at the edge of an
intermediate Internetwork. Each such ingress middlebox may break the
parcel out into smaller (sub-)parcels and encapsulate them in headers
suitable for traversing the Internetwork. These smaller parcels may
then be coalesced into one or more larger parcels at an egress
middlebox which either delivers them locally or forwards them further
over parcel-capable IP links toward the final destination. Middlebox
repackaging of parcels is therefore possible, making reordering and
even loss of individual segments possible. But, what matters is that
the number of parcels delivered to the final destination should be
kept to a minimum for the sake of efficiency, and that loss or
receipt of individual segments (and not parcel size) determines the
retransmission unit.
The following sections discuss rationale for creating and shipping
parcels as well as the actual protocol constructs and procedures
involved. IP parcels provide an essential building block for
accommodating larger Maximum Transmission Units (MTUs) in the
Internet. It is further expected that the parcel concept may drive
future innovation in applications, operating systems, network
equipment and data links.
2. Terminology
A "parcel" is defined as "a thing or collection of things wrapped in
paper in order to be carried or sent by mail". Indeed, there are
many examples of parcel delivery services worldwide that provide an
essential transit backbone for efficient business and consumer
transactions.
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In this same spirit, an "IP parcel" is simply a collection of up to
64 upper layer protocol segments wrapped in an efficient package for
transmission and delivery (i.e., a "packet-of-packets") while a
"singleton IP parcel" is simply a parcel that contains a single
segment. IP parcels are distinguished from ordinary packets through
the special header constructions discussed in this document.
The IP parcels construct is defined for both IPv4 and IPv6. Where
the document refers to "IPv4 header length", it means the total
length of the base IPv4 header plus all included options, i.e., as
determined by consulting the Internet Header Length (IHL) field.
Where the document refers to "IPv6 header length", however, it means
only the length of the base IPv6 header (i.e., 40 octets), while the
length of any extension headers is referred to separately as the
"extension header length". Finally, the term "IP header plus
extensions" refers generically to an IPv4 header plus all included
options or an IPv6 header plus all included extension headers.
When the document refers to "upper layer header length", it means the
length of either the UDP header (8 octets) or the TCP header plus
options (20 octets or more). It is important to note that only a
single IP header and a single (full) TCP/UDP header appears in each
parcel regardless of the number of segments included. This
distinction often provides a significant savings in overhead made
possible only by parcels.
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. Background and Motivation
Studies have shown that applications can realize greater performance
by sending and receiving larger packets due to reduced numbers of
system calls and interrupts as well as larger atomic data copies
between kernel and user space. Large packets also result in reduced
numbers of network device interrupts and better network utilization
in comparison with smaller packet sizes.
A first study [QUIC] involved performance enhancement of the QUIC
protocol [RFC9000] using the linux Generic Segment/Receive Offload
(GSO/GRO) facility. GSO/GRO provide a robust (but non-standard)
service very similar in nature to the IP parcel service described
here, and its application has shown significant performance increases
due to the increased transfer unit size between the operating system
kernel and QUIC application.
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A second study [I-D.templin-dtn-ltpfrag] showed that GSO/GRO also
improved performance for the Licklider Transmission Protocol (LTP)
[RFC5326] for small- to medium-sized segments. Historically, the NFS
protocol also saw significant performance increases using larger
(single-segment) UDP datagrams even when IP fragmentation is invoked,
and LTP still follows this profile today. Moreover, LTP shows this
(single-segment) performance increase profile extending to the
largest possible segment size which suggests that additional
performance gains may be possible using (multi-segment) IP parcels
that exceed 65535 octets.
TCP also benefits from larger packet sizes and efforts have
investigated TCP performance using jumbograms internally with changes
to the linux GSO/GRO facilities [BIG-TCP]. The idea is to use the
jumbo payload internally and to allow GSO/GRO to use buffer sizes
larger than 65535 octets, but with the understanding that links that
support jumbos natively are not yet widely available. Hence, IP
parcels provides a packaging that can be considered in the near term
under current deployment limitations.
A limiting consideration for sending large packets is that they are
often lost at links with smaller Maximum Transmission Units (MTUs),
and the resulting Packet Too Big (PTB) message may be lost somewhere
in the path back to the original source. This "Path MTU black hole"
condition can degrade performance unless robust path probing
techniques are used, however the best case performance always occurs
when no packets are lost due to size restrictions.
These considerations therefore motivate a design where transport
protocols should employ a maximum segment size no larger than 65535
octets (minus headers), while parcels that carry the segments may
themselves be significantly larger. Then, even if a middlebox needs
to sub-divide the parcels into smaller sub-parcels to forward further
toward the final destination, an important performance optimization
for the original source, final destination and network middleboxes
can be realized.
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An analogy: when a consumer orders 50 small items from a major online
retailer, the retailer does not ship the order in 50 separate small
boxes. Instead, the retailer puts as many of the small items as
possible into one or a few larger boxes (i.e., parcels) then places
the parcels on a semi-truck or airplane. The parcels may then pass
through one or more regional distribution centers where they may be
repackaged into different parcel configurations and forwarded further
until they are finally delivered to the consumer. But most often,
the consumer will only find one or a few parcels at their doorstep
and not 50 separate small boxes. This flexible parcel delivery
service greatly reduces shipping and handling cost for all including
the retailer, regional distribution centers and finally the consumer.
4. IP Parcel Formation
IP parcel formation is invoked by an upper layer protocol (identified
by the 5-tuple described above) when it prepares a data buffer
containing the concatenation of up to 64 segments. All non-final
segments MUST be equal in length while the final segment MUST NOT be
larger and MAY be smaller. Each non-final segment MUST NOT be larger
than 65535 octets minus the length of the TCP/UDP header and IPv4
header or IPv6 extension headers, minus the length of an additional
IPv6 header in case an encapsulation middlebox is visited on the path
(see: Section 7). The upper layer protocol then presents the buffer
and non-final segment size to the IP layer which appends a single IP
header plus extensions, a single full TCP/UDP header for the first
segment and a separate "shim" TCP/UDP header for each additional
segment before presenting the parcel either to an adaptation layer
interface or directly to an ordinary network interface without
engaging the adaptation layer (see: Section 7).
For IPv4, the IP layer prepares the parcel by appending an IPv4
header with a Jumbo Payload option formed as follows:
+--------+--------+--------+--------+--------+--------+
|Opt Type|Opt Len | Jumbo Payload Length |
+--------+--------+--------+--------+--------+--------+
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The IPv4 Jumbo Payload option format is identical to that defined in
[RFC2675], except that the IP layer sets option type to '00001011'
and option length to '00000110' noting that the length distinguishes
this type from its deprecated use as the IPv4 "Probe MTU" option
[RFC1063]. The IP layer then sets "Jumbo Payload Length" to the
lengths of the IPv4 header plus the combined length of all
concatenated segments (i.e., as a 32-bit value in network byte
order). The IP layer next sets the IPv4 header DF bit to 1, then
sets the IPv4 header Total Length field to the length of the first
segment only. Note that the IP layer can form true IPv4 jumbograms
(as opposed to parcels) by instead setting the IPv4 header Total
Length field to 0 (see: Section 11).
For IPv6, the IP layer forms a parcel by appending an IPv6 header
with a Hop-by-Hop Options extension header containing a Jumbo Payload
option formatted the same as for IPv4 above, but with option type set
to '11000010' and option length set to '00000100'. The IP layer then
sets "Jumbo Payload Length" to the lengths of all IPv6 extension
headers present plus the combined length of all concatenated
segments. The IP layer next sets the IPv6 header Payload Length
field to the length of the first segment only. Note that the IP
layer can form true IPv6 jumbograms (as opposed to parcels) by
instead setting the IPv6 header Payload Length field to 0 (see:
[RFC2675]).
An IP parcel therefore has the following structure:
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+--------+--------+--------+--------+
| |
~ Segment J (K octets) ~
| (UDP/TCP "shim") |
+--------+--------+--------+--------+
~ ~
~ ~
+--------+--------+--------+--------+
| |
~ Segment 3 (L octets) ~
| (UDP/TCP "shim") |
+--------+--------+--------+--------+
| |
~ Segment 2 (L octets) ~
| (UDP/TCP "shim") |
+--------+--------+--------+--------+
| |
~ Segment 1 (L octets) ~
|(Full UDP/TCP header plus options) |
+--------+--------+--------+--------+
| IP Header Plus Extensions |
~ {Total, Payload} Length = M ~
| Jumbo Payload Length = N |
+--------+--------+--------+--------+
where J is the total number of segments (between 1 and 64), L is the
length of each non-final segment which MUST NOT be larger than 65535
octets (minus headers as above) and K is the length of the final
segment which MUST NOT be larger than L. The value M is then set to
L if there are multiple segments or K if there is only a single
segment. Finally, the value N is set to the length of the full UDP
or TCP header (plus options), plus the length of the IP header plus
extensions for IPv4 or to the length of the extension headers only
for IPv6. The value N is then further calculated as follows:
N = N + (((J-1) * (L + shim_length)) + K)
Note: a "singleton" parcel is one that includes only the {TCP,
UDP}/IP headers plus extensions with J=1 and a single segment of
length K, while a "null" parcel is a singleton with (J=1; K=0), i.e.,
a parcel consisting of only the IP header plus extensions with no
octets beyond.
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5. UDP Parcels
A UDP Parcel is an IP Parcel that includes a full UDP header
immediately following the IP header plus extensions. The UDP header
is then followed by J segments prepared by the transport layer user
of UDP, where the first segment begins with a transport-specific
start delimiter (e.g., a sequence number field) and each non-first
segment begins with a "shim" UDP header including only the 2-octet
checksum field followed by the start delimiter. The length of each
segment is determined by the IP header {Total, Payload} length field
as discussed above.
The UDP Parcel is prepared in a similar fashion as for UDP jumbograms
[RFC2675], except that the UDP checksum for each segment is
calculated independently and written into the full/shim UDP header
checksum fields (while using the full UDP header for checksum
calculation for all segments). The same as for UDP jumbograms, the
full UDP header length field is set to 0.
6. TCP Parcels
A TCP Parcel is an IP Parcel that includes a full TCP header (plus
options) immediately following the IP header plus extensions. The
TCP header is then followed by J segments, where each non-first
segment begins with a "shim" TCP header including only the 2-octet
checksum field followed by a 4-octet sequence number field that
encodes the starting (TCP) sequence number for this segment. The
length of each segment is determined by the IP header {Total,
Payload} length field as discussed above.
The TCP Parcel is prepared in a similar fashion as for TCP jumbograms
[RFC2675], except that the TCP checksum for each segment is
calculated independently and written into the full/shim TCP header
checksum fields (while using the full TCP header for checksum
calculation for all segments).
7. Transmission of IP Parcels
The IP layer next presents the parcel to the outgoing network
interface. For ordinary IP interfaces, the interface simply forwards
the parcel over the underlying link the same as for any IP packet
after which it may then be forwarded by any number of routers over
additional consecutive parcel-capable IP links. If any next hop IP
link in the path either does not support parcels or configures an MTU
that is too small to transit the parcel without fragmentation, the
router instead opens the parcel and forwards each enclosed segment as
a separate IP packet. The router forwards each segment by appending
a copy of the parcel's IP header to each segment but with the Jumbo
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Payload option removed according to the standards [RFC0791][RFC8200])
and also replacing each "shim" TCP/UDP header with a copy of the full
TCP/UDP header while changing the sequence number and checksum to the
"shim" values as necessary. Or, if the router does not recognize
parcels at all, it drops the parcel and may return an ICMP "Parameter
Problem" message.
If the outgoing network interface is an OMNI interface
[I-D.templin-6man-omni], the OMNI Adaptation Layer (OAL) of this
First Hop Segment (FHS) OAL source node forwards the parcel to the
next OAL hop which may be either an OAL intermediate node or a Last
Hop Segment (LHS) OAL destination node (which may also be the final
destination itself). The OAL source assigns a monotonically-
incrementing (modulo 127) "Parcel ID" and subdivides the parcel into
sub-parcels no larger than the maximum of the path MTU to the next
hop or 65535 octets (minus headers) by determining the number of
segments of length L that can fit into each sub-parcel under these
size constraints. For example, if the OAL source determines that a
sub-parcel can contain 3 segments of length L, it creates sub-parcels
with the first containing segments 1-3, the second containing
segments 4-6, etc. and with the final containing any remaining
segments. The OAL source then appends identical {TCP, UDP}/IP
headers plus extensions to each sub-parcel while resetting M and N in
each according to the above equations with J set to 3 (and K = L) for
each non-final sub-parcel and with J set to the remaining number of
segments for the final sub-parcel.
The OAL source next performs IP encapsulation on each sub-parcel with
destination set to the next hop IP address then inserts an IPv6
Fragment Header after the IP encapsulation header, i.e., even if the
encapsulation header is IPv4, even if no actual fragmentation is
needed and/or even if the Jumbo Payload option is present. The OAL
source then assigns an appropriate 32-bit Identification number that
is monotonically-incremented for each consecutive sub-parcel, then
performs IPv6 fragmentation over the sub-parcel if necessary to
create fragments small enough to traverse the path to the next OAL
hop while writing the Parcel ID and setting or clearing the "Parcel
(P)" and "(More) Sub-Parcels (S)" bits in the Fragment Header of the
first fragment (see: [I-D.templin-6man-fragrep]). (The OAL source
sets P to 1 for a parcel or to 0 for a non-parcel. When P is 1, the
OAL source next sets S to 1 for non-final sub-parcels or to 0 if the
sub-parcel contains the final segment.) The OAL source then forwards
each IP encapsulated packet/fragment to the next OAL hop.
When the next OAL hop receives the encapsulated IP fragments or whole
packets, it validates the Identifications and reassembles if
necessary. If the P flag in the first fragment is 0, the next hop
then processes the reassembled entity as an ordinary IP packet;
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otherwise it continues processing as a sub-parcel. If the next hop
is an OAL intermediate node, it may retain the sub-parcels along with
their Parcel ID and Identification values for a brief time in hopes
of re-combining with peer sub-parcels of the same original parcel
identified by the 4-tuple consisting of the IP encapsulation (source,
destination, Identification, Parcel ID). The combining entails the
concatenation of the segments included in sub-parcels with the same
Parcel ID and with Identification values within 64 of one another to
create a larger sub-parcel possibly even as large as the entire
original parcel. Order of concatenation need not be strictly
maintained, with the exception that the final sub-parcel (i.e., the
one with S set to 0) must occur as the final concatenation before
transmission. The OAL intermediate node then appends a common {TCP,
UDP}/IP header plus extensions to each re-combined sub-parcel while
resetting M and N in each according to the above equations with J, K
and L set accordingly.
This OAL intermediate node next forwards the re-combined sub-
parcel(s) to the next hop toward the OAL destination using
encapsulation the same as specified above. (The intermediate node
MUST ensure that the S flag remains set to 0 in the sub-parcel that
contains the final segment.) When the sub-parcel(s) arrive at the
OAL destination, the OAL destination re-combines them into the
largest possible sub-parcels while honoring the S flag as above. If
the OAL destination is also the final destination, it delivers the
sub-parcels to the IP layer which acts on the enclosed 5-tuple
information supplied by the original source. Otherwise, the OAL
destination forwards each sub-parcel toward the final destination the
same as for an ordinary IP packet the same as discussed above.
Note: while the OAL destination and/or final destination could
theoretically re-combine the sub-parcels of multiple different
parcels with identical upper layer protocol 5-tuples and with non-
final segments of identical length, this process could become
complicated when the different parcels each have final segments of
diverse lengths. Since this might interfere with any perceived
performance advantages, the decision of whether and how to perform
inter-parcel concatenation is an implementation matter.
Note: some IPv6 fragmentation and reassembly implementations may
require a well-formed IPv6 header to perform their operations. When
the encapsulation is based on IPv4, such implementations translate
the encapsulation header into an IPv6 header with IPv4-Mapped IPv6
addresses before performing the fragmentation/reassembly operation,
then restore the original IPv4 header before further processing.
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Note: sub-dividing a larger parcel into two or more sub-parcels
entails the translation of the {TCP,UDP} "shim" header of the first
segment in each new sub-parcel into a full {TCP, UDP} header. For
TCP, the translation is based on copying the full TCP header from the
original parcel while replacing the sequence number and checksum
values with the shim header information. For UDP, the translation is
based on copying the full UDP header from the original parcel while
replacing only the checksum value. Note that the checksum values
found in the shim headers are still valid and need not be
recalculated.
Note: combining two or more sub-parcels into a larger parcel entails
the translation of each former sub-parcel's first segment full {TCP,
UDP} header into a {TCP, UDP} "shim" header. For TCP, the
translation is based on copying the sequence number and checksum
values into the shim header while discarding the full header. For
UDP, the translation is based on copying only the checksum value into
the shim header while discarding the full header. Note as above that
the checksum values need not be recalculated.
8. Parcel Path Qualification
To determine whether parcels are supported over at least a leading
portion of the forward path toward the final destination, the
original source can send a singleton IP parcel formatted as a "Parcel
Probe" that may include an upper layer protocol probe segment (e.g.,
a data segment, an ICMP Echo Request message, etc.). The purpose of
the probe is to elicit a "Parcel Reply" and possibly also an ordinary
upper layer protocol probe reply from the final destination.
If the original source receives a positive Parcel Reply, it marks the
path as "parcels supported" and ignores any ICMP [RFC0792][RFC4443]
and/or Packet Too Big (PTB) messages [RFC1191][RFC8201] concerning
the probe. If the original source instead receives a negative Parcel
Reply or no reply, it marks the path as "parcels not supported" and
may regard any ICMP and/or PTB messages concerning the probe (or its
contents) as indications of a possible path MTU restriction.
The original source can therefore send Parcel Probes in parallel with
sending real data as ordinary IP packets. If the original source
receives a positive Parcel Reply, it can begin using IP parcels.
Parcel Probes use the Jumbo Payload option type (see: Section 4) but
set a different option length and replace the option value with
control information plus a 4-octet "Path MTU" value into which
conformant middleboxes write the minimum link MTU observed in a
similar fashion as described in [RFC1063][I-D.ietf-6man-mtu-option].
Parcel Probes can also include an upper layer protocol probe segment,
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e.g., per [RFC4821][RFC8899]. When an upper layer protocol probe
segment is included, it appears immediately after the IP header plus
extensions and corresponds to the same 5-tuple values that appear in
ordinary data packets for this flow.
The original source sends Parcel Probes unidirectionally in the
forward path toward the final destination to elicit a Parcel Reply,
since it will often be the case that IP parcels are supported only in
the forward path and not in the return path. Parcel Probes may be
dropped in the forward path by any node that does not recognize IP
parcels, but a Parcel Reply must not be dropped even if IP parcels
are not recognized along portions of the return path. For this
reason, Parcel Probes are packaged as IPv4 (header) options or IPv6
Hop-by-Hop options while Parcel Replys are always packaged as IPv6
Destination Options (i.e., regardless of the IP protocol version).
Original sources send Parcel Probes and Replys that include a Jumbo
Payload option coded in an alternate format as follows:
+--------+--------+--------+--------+
|Opt Type|Opt Len | Nonce-1 |
+--------+--------+--------+--------+
| Nonce-2 |
+--------+--------+--------+--------+
| PMTU |
+--------+--------+--------+--------+
| Code | Check |
+--------+--------+
For IPv4, the original source includes the option as an IPv4 option
with Type set to '00001011' the same as for an ordinary IPv4 parcel
(see: Section 4) but with Length set to '00001110' to distinguish
this as a probe/reply. The original source sets Nonce-1 to 0xffff,
sets Nonce-2 to a (pseudo)-random 32-bit value and sets PMTU to the
MTU of the outgoing IPv4 interface. The original source then sets
Code to 0, sets Check to the same value that will appear in the TTL
of the outgoing IPv4 header, then finally sets IPv4 Total Length to
the lengths of the IPv4 header plus the upper layer protocol probe
segment (if any) and sends the Parcel Probe via the outgoing IPv4
interface. According to [RFC7126], middleboxes (i.e., routers,
security gateways, firewalls, etc.) that do not observe this
specification SHOULD drop IP packets that contain option type
'00001011' ("IPv4 Probe MTU") but some might instead either attempt
to implement [RFC1063] or ignore the option altogether. IPv4
middleboxes that observe this specification instead MUST process the
option as a Parcel Probe as specified below.
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For IPv6, the original source includes the probe option as an IPv6
Hop-by-Hop option with Type set to '11000010' the same as for an
ordinary IPv6 parcel (see: Section 4) but with Length set to
'00001100' to distinguish this as a probe. The original source sets
the concatenation of Nonce-1 and Nonce-2 to a (pseudo)-random 48-bit
value and sets PMTU to the MTU of the outgoing IPv6 interface. The
original source then sets Code to 0, sets Check to the same value
that will appear in the Hop Limit of the outgoing IPv6 header, then
finally sets IPv6 Payload Length to the lengths of the IPv6 extension
headers plus the upper layer protocol probe segment (if any) and
sends the Parcel Probe via the outgoing IPv6 interface. According to
[RFC2675], middleboxes (i.e., routers, security gateways, firewalls,
etc.) that recognize the IPv6 Jumbo Payload option but do not observe
this specification SHOULD return an ICMPv6 Parameter Problem message
(and presumably also drop the packet). IPv6 middleboxes that observe
this specification instead MUST process the option as a Parcel Probe
as specified below.
When a middlebox that observes this specification receives a Parcel
Probe it first compares the Check value with the IP header Hop Limit/
TTL; if the values differ, the middlebox MUST return a negative
Parcel Reply (see below) and drop the probe. Otherwise, if the next
hop IP link either does not support parcels or configures an MTU that
is too small to pass the probe, the middlebox compares the PMTU value
with the MTU of the inbound link for the probe and MUST (re)set PMTU
to the lower MTU. The middlebox then MUST return a positive Parcel
Reply (see below) and convert the probe into an ordinary IP packet by
removing the probe option according to [RFC0791] or [RFC8200]. If
the next hop IP link configures a sufficiently large MTU to pass the
packet, the middlebox then MUST forward the packet to the next hop;
otherwise, it MUST drop the packet and return a suitable PTB. If the
next hop IP link both supports parcels and configures an MTU that is
large enough to pass the probe, the middlebox instead compares the
probe PMTU value with the MTUs of both the inbound and outbound links
for the probe and MUST (re)set PMTU to the lower MTU. The middlebox
then MUST reset Check to the same value that will appear in the TTL/
Hop Limit of the outgoing IP header, and MUST forward the Parcel
Probe to the next hop.
The final destination may therefore receive either an ordinary IP
packet containing an upper layer protocol probe or a Parcel Probe.
If the final destination receives an ordinary IP packet, it performs
any necessary integrity checks then delivers the packet to upper
layers which will return an upper layer probe response. If the final
destination instead receives a Parcel Probe, it first compares the
Check value with the IP header Hop Limit/TTL; if the values differ,
the final destination MUST drop the probe and return a negative
Parcel Reply (see below). Otherwise, the final destination compares
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the probe PMTU value with the MTU of the inbound link and MUST
(re)set PMTU to the lower MTU. The final destination then MUST
return a positive Parcel Reply (see below) and convert the probe into
an ordinary IP packet by removing the Parcel Probe option according
to the standards [RFC0791][RFC8200].The final destination then
performs any necessary integrity checks and delivers the packet to
upper layers.
When the middlebox or final destination returns a Parcel Reply, it
prepares an IP header of the same protocol version that appeared in
the Parcel Probe with source and destination addresses reversed, with
{Protocol, Next Header} set to the value '60' (i.e., "IPv6
Destination Option") and with an IPv6 Destination Option header with
Next Header set to the value '59' (i.e., "IPv6 No Next Header")
[RFC8200]. The node next copies the body of the Parcel Probe option
as the sole Parcel Reply Destination Option (and for IPv4 resets Type
to '11000010' and Length to '00001100') and includes no other octets
beyond the end of the option. The node then MUST (re)set Check to 1
for a positive or to 0 for a negative Parcel Reply, then MUST finally
set the IP header {Total, Payload} Length field according to the
length of the included Destination Option and return the Parcel Reply
to the source. (Since filtering middleboxes may drop IPv4 packets
with Protocol '60' the destination MUST wrap an IPv4 Parcel Reply in
UDP/IPv4 headers with the IPv4 source and destination addresses
copied from the Parcel Reply and with UDP port numbers set to the UDP
port number for OMNI [I-D.templin-6man-omni].)
After sending a Parcel Probe the original source may therefore
receive a Parcel Reply (see above) and/or an upper layer protocol
probe reply. If the source receives a Parcel Reply, it first matches
Nonce-2 (and for IPv6 only also matches Nonce-1) with the values it
had included in the Parcel Probe. If the values do not match, the
source discards the Parcel Reply. Next, the source examines the
Check value and marks the path as "parcels supported" if the value is
1 or "parcels not supported" otherwise. If the source marks the path
as "parcels supported", it also records the PMTU value as the maximum
parcel size for the forward path to this destination.
After receiving a positive Parcel Reply, the original source can
begin sending IP parcels addressed to the final destination up to the
size recorded in the PMTU. Any upper layer protocol probe replies
will determine the maximum segment size that can be included in the
parcel, but this is an upper layer consideration. The original
source should then periodically re-initiate Parcel Path Qualification
as long as it continues to forward parcels toward the final
destination (i.e., in case the forward path fluctuates). If at any
time performance appears to degrade, the original source should cease
sending IP parcels and/or re-initiate Parcel Path Qualification.
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Note: For IPv4, the original source sets the Parcel Probe Nonce-1
field to 0xffff on transmission and ignores the Nonce-1 field value
in any corresponding Parcel Replys. This avoids any possible
confusion in case an IPv4 router on the path rewrites the Nonce-1
field in a wayward attempt to implement [RFC1063].
Note: The PMTU value returned in a positive Parcel Reply determines
only the maximum IP parcel size for the path, while the maximum upper
layer protocol segment size may be significantly smaller. The upper
layer protocol segment size is instead determined separately
according to any upper layer protocol probes and must be assumed to
be no larger than 1/64th of the maximum IP parcel size unless a
larger size is discovered by probing.
Note: Parcel probes should include an (expendable) segment of the
same upper layer protocol 5-tuple that would be used to transport
ordinary data packets. This ensures that the probes will travel over
the same paths as for ordinary data packets.
9. Integrity
Each segment of a (multi-segment) IP parcel includes its own upper
layer protocol integrity check. This means that IP parcels can
support stronger integrity for the same amount of upper layer
protocol data in comparison with an ordinary IP packet or Jumbogram
containing only a single segment. The integrity checks must then be
verified at the final destination, which accepts any segments with
correct integrity while discarding all other segments and counting
them as a loss event.
IP parcels can range in length from as small as only the IP headers
themselves to as large as the IP headers plus (64 * (65535 minus
headers)) octets. Although link layer integrity checks provide
sufficient protection for contiguous data blocks up to approximately
9KB, reliance on the presence of link-layer integrity checks may not
be possible over links such as tunnels. Moreover, the segment
contents of a received parcel may arrive in an incomplete and/or
rearranged order with respect to their original packaging.
For these reasons, the OAL at each hop of an OMNI link includes an
integrity check when it performs IP fragmentation on a sub-parcel,
with the integrity verified during reassembly at the next hop.
10. RFC2675 Updates
Section 3 of [RFC2675] provides a list of certain conditions to be
considered as errors. In particular:
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error: IPv6 Payload Length != 0 and Jumbo Payload option present
error: Jumbo Payload option present and Jumbo Payload Length <
65,536
Implementations that obey this specification ignore these conditions
and do not consider them as errors.
11. IPv4 Jumbograms
By defining a new IPv4 Jumbo Payload option, this document also
implicitly enables a true IPv4 jumbogram service defined as an IPv4
packet with a Jumbo Payload option included and with Total Length set
to 0. All other aspects of IPv4 jumbograms are the same as for IPv6
jumbograms [RFC2675].
12. Implementation Status
Common widely-deployed implementations include services such as TCP
Segmentation Offload (TSO) and Generic Segmentation/Receive Offload
(GSO/GRO). These services support a robust (but not standardized)
service that has been shown to improve performance in many instances.
Implementation of the IP parcel service is a work in progress.
13. IANA Considerations
The IANA is instructed to change the "MTUP - MTU Probe" entry in the
'ip option numbers' registry to the "JUMBO - IPv4 Jumbo Payload"
option. The Copy and Class fields must both be set to 0, and the
Number and Value fields must both be set to 11'. The reference must
be changed to this document [RFCXXXX].
14. Security Considerations
Original sources match the Nonce values in received Parcel Replies
with their corresponding Parcel Probes. If the values match, the
Parcel Reply is likely an authentic response to the Parcel Probe. In
environments where stronger authentication is necessary, the message
authentication services of OMNI can be applied
[I-D.templin-6man-omni].
Multi-layer security solutions may be necessary to ensure
confidentiality, integrity and availability in some environments.
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15. Acknowledgements
This work was inspired by ongoing AERO/OMNI/DTN investigations. The
concepts were further motivated through discussions on the intarea
and 6man lists.
A considerable body of work over recent years has produced useful
"segmentation offload" facilities available in widely-deployed
implementations.
.
16. References
16.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
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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>.
[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>.
16.2. Informative References
[BIG-TCP] Dumazet, E., "BIG TCP, Netdev 0x15 Conference (virtual),
https://netdevconf.info/0x15/session.html?BIG-TCP", 31
August 2021.
[I-D.ietf-6man-mtu-option]
Hinden, R. M. and G. Fairhurst, "IPv6 Minimum Path MTU
Hop-by-Hop Option", Work in Progress, Internet-Draft,
draft-ietf-6man-mtu-option-15, 10 May 2022,
<https://www.ietf.org/archive/id/draft-ietf-6man-mtu-
option-15.txt>.
[I-D.ietf-tcpm-rfc793bis]
Eddy, W. M., "Transmission Control Protocol (TCP)
Specification", Work in Progress, Internet-Draft, draft-
ietf-tcpm-rfc793bis-28, 7 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-tcpm-
rfc793bis-28.txt>.
[I-D.templin-6man-fragrep]
Templin, F. L., "IPv6 Fragment Retransmission and Path MTU
Discovery Soft Errors", Work in Progress, Internet-Draft,
draft-templin-6man-fragrep-07, 29 March 2022,
<https://www.ietf.org/archive/id/draft-templin-6man-
fragrep-07.txt>.
[I-D.templin-6man-omni]
Templin, F. L., "Transmission of IP Packets over Overlay
Multilink Network (OMNI) Interfaces", Work in Progress,
Internet-Draft, draft-templin-6man-omni-68, 1 July 2022,
<https://www.ietf.org/archive/id/draft-templin-6man-omni-
68.txt>.
[I-D.templin-dtn-ltpfrag]
Templin, F. L., "LTP Fragmentation", Work in Progress,
Internet-Draft, draft-templin-dtn-ltpfrag-08, 1 February
2022, <https://www.ietf.org/archive/id/draft-templin-dtn-
ltpfrag-08.txt>.
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[QUIC] Ghedini, A., "Accelerating UDP packet transmission for
QUIC, https://blog.cloudflare.com/accelerating-udp-packet-
transmission-for-quic/", 8 January 2020.
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, DOI 10.17487/RFC1063,
July 1988, <https://www.rfc-editor.org/info/rfc1063>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[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>.
[RFC5326] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
Transmission Protocol - Specification", RFC 5326,
DOI 10.17487/RFC5326, September 2008,
<https://www.rfc-editor.org/info/rfc5326>.
[RFC7126] Gont, F., Atkinson, R., and C. Pignataro, "Recommendations
on Filtering of IPv4 Packets Containing IPv4 Options",
BCP 186, RFC 7126, DOI 10.17487/RFC7126, February 2014,
<https://www.rfc-editor.org/info/rfc7126>.
[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>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
United States of America
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Email: fltemplin@acm.org
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