IP Parcels and Advanced JumbosBoeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftIP packets (both IPv4 and IPv6) contain a single unit of transport
layer protocol data which becomes the retransmission unit in case of loss.
Transport layer protocols including the Transmission Control Protocol (TCP)
and reliable transport protocol users of the User Datagram Protocol (UDP)
prepare data units known as segments which the network layer packages into
individual IP packets each containing only a single segment. This document
presents new constructs known as IP Parcels and Advanced Jumbos. IP parcels
permit a single packet to include multiple segments as a "packet-of-packets",
while advanced jumbos offer significant operational advantages over basic
jumbograms for transporting truly large singleton segments. IP parcels and
advanced jumbos provide essential building blocks for improved performance,
efficiency and integrity while encouraging larger Maximum Transmission
Units (MTUs) in the Internet.IP packets (both IPv4 and IPv6 ) contain a single unit of transport layer protocol
data which becomes the retransmission unit in case of loss. Transport
layer protocols such as the Transmission Control Protocol (TCP) and reliable transport protocol users of the User
Datagram Protocol (UDP) (including QUIC
, LTP and others)
prepare data units known as segments which the network layer packages
into individual IP packets each containing only a single segment. This
document presents a new construct known as the IP Parcel which permits
a single packet to include multiple segments. The parcel is essentially
a "packet-of-packets" with the full {TCP,UDP}/IP headers appearing
only once but with possibly multiple segments included.Transport layer protocol entities form parcels by preparing a
data buffer (or buffer chain) of consecutive transport layer
protocol segments that can be broken out into individual packets
and/or smaller sub-parcels if necessary. All segments except the
final one must be equal in length and no larger than 65535 octets,
while the final segment must be no larger than the others. The
transport layer protocol entity then delivers the buffer(s),
number of segments and non-final segment size to the network
layer which merges the segments into the body of a parcel. The
network layer finally appends an Integrity Block, a {TCP,UDP}
header and an IP header plus extensions that identify this as
a parcel and not an ordinary packet.The network layer then forwards each parcel over consecutive
parcel-capable links in a path until they arrive at a node with
a next hop link that does not support parcels, a parcel-capable
link with a size restriction, or an ingress Overlay Multilink
Network (OMNI) Interface
connection to an OMNI link that spans intermediate Internetworks.
In the first case, the original source or next hop router applies
packetization to break the parcel into individual IP packets. In
the second case, the source/router applies network layer
parcellation to form smaller sub-parcels. In the final case,
the OMNI interface applies adaptation layer parcellation to
form still smaller sub-parcels, then applies adaptation layer
IPv6 encapsulation and fragmentation if necessary. The node
then forwards the resulting packets/parcels/fragments to the
next hop.Following IPv6 reassembly if necessary, an egress OMNI
interface applies adaptation layer reunification if necessary
to merge multiple sub-parcels into a minimum number of larger
(sub-)parcels then delivers them to the network layer which either
processes them locally or forwards them via the next hop link
toward the final destination. The final destination can then apply
network layer (parcel-based) reunification or (packet-based)
restoration if necessary to deliver a minimum number of larger
(sub-)parcels to the transport layer. Reordering, loss or corruption
of individual segments within the network is therefore possible, but
most importantly the parcels delivered to the final destination's
transport layer should be the largest practical size for best
performance, and loss or receipt of individual segments (rather
than parcel size) determines the retransmission unit.This document further introduces an Advanced Jumbo service that
provides useful extensions beyond the basic IPv6 jumbogram service
defined in . Advanced jumbos are defined for
both IP protocol versions and provide end systems and routers with a
more robust service when the transmission of truly large singleton
segments is necessary.The following sections discuss rationale for creating and shipping
IP parcels and advanced jumbos as well as actual protocol constructs
and procedures involved. IP parcels and advanced jumbos provide
essential building blocks for improved performance, efficiency and
integrity while encouraging larger Maximum Transmission Units (MTUs).
These services should further inspire future innovation in applications,
transport protocols, operating systems, network equipment and data
links in ways that promise to transform the Internet architecture.The Oxford Languages dictionary defines a "parcel" 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.In this same spirit, an "IP parcel" is simply a collection of at most
64 transport layer protocol segments wrapped in an efficient package
for transmission and delivery, i.e., a "packet-of-packets". IP parcels
are distinguished from ordinary packets and jumbograms through the
constructs specified in this document.The IP parcel 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 "IPv6 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.The term "advanced jumbo" refers to a new type of IP jumbogram defined
for both IP protocol versions and derived from the basic IPv6 jumbogram
construct defined in . Advanced jumbos include a
32-bit Jumbo Payload Length field the same as for basic IPv6 jumbograms,
but are differentiated from parcels and other jumbogram types by including
a "Jumbo Type" value '1' in the IP {Total, Payload} Length field.Where the document refers to "{TCP,UDP} header length", it means
the length of either the TCP header plus options (20 or more octets)
or the UDP header (8 octets). 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 overhead savings advantage
made possible only by IP parcels.Where the document refers to checksum calculations, it means the
standard Internet checksum unless otherwise specified. The same as for
TCP , UDP and IPv4
, the standard Internet checksum is defined as
(sic) "the 16-bit one's complement of the one's complement sum of all
(pseudo-)headers plus data, padded with zero octets at the end (if
necessary) to make a multiple of two octets". A notional Internet
checksum algorithm can be found in , while
practical implementations require detailed attention to network byte
ordering to ensure interoperability between diverse architectures.The terms "application layer (L5 and higher)", "transport layer
(L4)", "network layer (L3)", "(data) link layer (L2)" and "physical
layer (L1)" are used consistently with common Internetworking
terminology, with the understanding that reliable delivery protocol
users of UDP are considered as transport layer elements. The OMNI
specification further defines an "adaptation layer" logically positioned
below the network layer but above the link layer (which may include
physical links and Internet- or higher-layer tunnels). The adaptation
layer is not associated with a layer number itself and is simply known
as "the layer below L3 but above L2". A network interface is a node's
attachment to a link (via L2), and an OMNI interface is therefore
a node's attachment to an OMNI link (via the adaptation layer). The term "parcel/jumbo-capable link/path" refers to paths that transit
interfaces to adaptation layer and/or link layer media (either physical or
virtual) capable of transiting {TCP,UDP}/IP packets that employ the
parcel/jumbo constructs specified in this document. The source and each
router in the path has a "next hop link" that forwards parcels/jumbos
toward the final destination, while each router and the final destination
has a "previous hop link" that accepts en route parcels/jumbos. Each next
hop link must be capable of forwarding parcels/jumbos (after first applying
parcellation if necessary) with segment lengths no larger than can transit
the link. Currently only the OMNI link satisfies these properties, while
other link types that support parcels/jumbos should soon follow.The term "5-tuple" refers to a transport layer protocol entity
identifier that includes the network layer (source address,
destination address, source port, destination port, protocol number).
The term "3-tuple" refers to a network layer parcel entity
identifier that includes the adaptation layer (source address,
destination address, Parcel ID).The Internetworking term "Maximum Transmission Unit (MTU)" is
widely understood to mean the largest packet size that can transit
a single link ("link MTU") or an entire path ("path MTU") without
requiring network layer IP fragmentation. If the MTU value returned
during parcel path qualification is larger than 65535 (plus the
length of the parcel headers), it determines the maximum-sized
parcel or jumbo that can transit the link/path without requiring a
router to perform packetization/parcellation. If the MTU is 65535
or smaller, the value instead determines the "Maximum Segment Size
(MSS)" for the leading portion of the path up to a router that
cannot forward the parcel further. (Note that this size may still
be larger than the MSS that can transit the remainder of the path
to the final destination, which can only be determined through
explicit MSS probing.)The terms "packetization" and "restoration" refer to a network
layer process in which the original source or a router on the path
breaks a parcel out into individual IP packets that can transit
the remainder of the path without loss due to a size restriction.
The final destination then restores the combined packet contents
into a parcel before delivery to the transport layer. In current
practice, packetization/restoration can be considered as functional
equivalents to the well-known Generic Segmentation/Receive Offload
(GSO/GRO) services.The terms "parcellation" and "reunification" refer to either
network layer or adaptation layer processes in which the original
source or a router on the path breaks a parcel into smaller
sub-parcels that can transit the path without loss due to a size
restriction. These sub-parcels are then reunified into larger
(sub-)parcels before delivery to the transport layer. As a network
layer process, the sub-parcels resulting from parcellation may
only be reunified at the final destination. As an adaptation
layer process, the resulting sub-parcels may be first reunified
at an adaptation layer egress node then possibly further
reunified by the network layer of the final destination.The terms "fragmentation" and "reassembly" follow exactly from
their definitions in the IPv4 and IPv6
standards. In particular, OMNI interfaces
support IPv6 encapsulation and fragmentation as an adaptation
layer process that can transit packets or (sub-)parcels of sizes
that exceed the underlying Internetwork path MTU. OMNI
fragmentation/reassembly occurs at a lower layer of the
protocol stack than restoration and/or reunification and
therefore provides a complimentary service."Automatic Extended Route Optimization (AERO)" and the "Overlay Multilink Network
Interface (OMNI)" provide an
adaptation layer framework for transmission of IP parcels and advanced
jumbos over one or more concatenated Internetworks. AERO/OMNI will
provide an operational environment for IP parcels beginning from the
earliest deployment phases and extending indefinitely to accommodate
continuous future growth. As more and more parcel/jumbo-capable links
are deployed (e.g., in data centers, edge networks, space-domain, and
other high data rate services) AERO/OMNI will continue to provide an
essential service for Internetworking performance maximization.The parcel sizing variables "J", "K", "L" and "M" are cited
extensively throughout the document. "J" denotes the number of
non-final segments included in the parcel, "K" is the length of
the final segment, "L" is the length of each non-final segment
and "M" is termed the "Parcel Payload Length".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
when, and only when,
they appear in all capitals, as shown here.Studies have shown that applications can improve their 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. Larger packets also result in reduced numbers of network
device interrupts and better network utilization (e.g., due to header
overhead reduction) in comparison with smaller packets.A first study involved performance enhancement
of the QUIC protocol using the linux Generic
Segment/Receive Offload (GSO/GRO) facility. GSO/GRO provides a robust
service that has shown significant performance increases based on a
multi-segment transfer capability between the operating system kernel
and QUIC applications. GSO/GRO performs (virtual) fragmentation and
reassembly at the transport layer with the transport protocol segment
size limited by the path MTU (typically 1500 octets or smaller in
today's Internet).A second study showed that
GSO/GRO also improves performance for the Licklider Transmission
Protocol (LTP) used for the Delay Tolerant
Networking (DTN) Bundle Protocol for segments
larger than the actual path MTU through the use of OMNI interface
encapsulation and fragmentation. 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 are possible
using (multi-segment) IP parcels that approach or even exceed
65535 octets in total length.TCP also benefits from larger packet sizes and efforts have
investigated TCP performance using jumbograms internally with changes
to the linux GSO/GRO facilities . The approach
proposed to use the Jumbo Payload option internally and to allow GSO/GRO
to use buffer sizes larger than 65535 octets, but with the understanding
that links that support jumbograms natively are not yet widely available.
Hence, IP parcels provide 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 MTU restrictions, and the resulting Packet Too
Big (PTB) messages may
be lost somewhere in the return path 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 loss of packets due to size restrictions is minimized.These considerations therefore motivate a design where transport
protocols can employ segment sizes as large as 65535 octets while
parcels that carry multiple segments may themselves be significantly
larger. This would allow the receiving transport layer protocol entity
to process multiple segments in parallel instead of one at a time per
existing practices. Parcels therefore support improvements in
performance, integrity and efficiency for the original source, final
destination and networked path as a whole. This is true even if the
network and lower layers need to apply packetization/restoration,
parcellation/reunification and/or fragmentation/reassembly.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 packs 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.A transport protocol entity identified by its 5-tuple
forms a parcel body by preparing a data buffer (or buffer chain)
containing at most 64 transport layer protocol segments, with each
TCP non-first segment preceded by a 4-octet Sequence Number header.
All non-final segments MUST be equal in length while the final
segment MUST NOT be larger and MAY be smaller. The number of
non-final segments is represented as J; therefore the total
number of segments is represented as (J + 1).The non-final segment size L is set to a 16-bit value that
MUST be no smaller than 256 octets and SHOULD be no larger than
65535 octets minus the length of the {TCP,UDP} header (plus options),
minus the length of the IP header (plus options/extensions),
minus 2 octets for the per-segment Checksum (see: ). The final segment length K MUST NOT be larger
than L but MAY be smaller. The transport layer protocol entity
then presents the buffer(s) and size L to the network layer,
noting that the combined buffer length(s) may exceed 65535
octets when there are sufficient segments of a large enough
size.If the next hop link is not parcel capable, the network layer
performs packetization to package each segment as an individual IP
packet as discussed in . If the next
hop link is parcel capable, the network layer instead completes the
parcel by appending an Integrity Block of 2-octet per-segment Checksums,
a single full {TCP,UDP} header (plus options) and a single full IP
header (plus options/extensions). The network layer finally includes
a specially-formatted Parcel Payload option as an extension to the
IP header of each parcel prior to transmission over a network
interface.The Parcel Payload option format for both IP protocol versions
appears as shown in :
For IPv4, the network layer includes the Parcel Payload option
as an IPv4 header option with option-type set to '11' and
option-length set to '8'. (Note: the length also distinguishes
this type from its obsoleted use as the IPv4 Probe MTU option
.) The network layer sets Code to '255'
and sets Check to the same value that will appear in the IPv4 header
TTL field upon transmission to the next hop. The network layer also
sets Parcel Payload Length to a 3-octet value M that encodes the
length of the IPv4 header plus the length of the {TCP,UDP} header
plus the combined length of the Integrity Block plus all
concatenated segments. The network layer then sets the IPv4
header DF bit to '1' and Total Length field to the non-final
segment size L.For IPv6, the network layer includes the Parcel Payload option
as an IPv6 Hop-by-Hop Option with Option Type set to 'C2' (hexadecimal)
and Option Data Length set to '4' the same as for the IPv6 Jumbo
Payload Option (for further Hop-by-Hop
option processing considerations, see: ). The network layer also
sets Parcel Payload Length to a 3-octet value M that encodes the
lengths of all IPv6 extension headers present plus the length of
the {TCP,UDP} header plus the combined length of the Integrity
Block plus all concatenated segments. The network layer then
sets the IPv6 header Payload Length field to L.For both IP protocol versions, the network layer then sets
Index to an ordinal segment index value between '0' and '63',
sets the "(P)arcel" flag to '1' and sets the "More (S)egments"
flag to '1' for non-final sub-parcels or '0' for the final (sub-)parcel.
(Note that Index values other than '0' identify the initial segment
index in non-first sub-parcels of a larger original parcel,
whereas first (sub-)parcels always set Index to '0'.)Following this transport and network layer processing,
{TCP,UDP}/IP parcels therefore have the structures shown in
:The {TCP,UDP}/IP header is immediately followed by an Integrity
Block containing (J + 1) 2-octet Checksums concatenated in numerical
order as shown in :
The Integrity Block is then followed by (J + 1) transport
layer segments. For TCP, the TCP header Sequence Number field
encodes a 4-octet starting sequence number for the first segment
only, while each additional segment is preceded by its own 4-octet
Sequence Number field. For this reason, the length of the first
segment is only (L-4) octets since the 4-octet TCP header
Sequence Number field applies to that segment. (All non-first
TCP segments instead begin with their own Sequence Number
headers, with the 4-octet length included in L and K.)Note: The per-segment Checksums appear in a contiguous Integrity Block
immediately following the {TCP,UDP}/IP headers instead of inline with
the parcel segments to greatly increase the probability that they will
appear in the contiguous head of a kernel receive buffer even if the
parcel was subject to OMNI interface IPv6 fragmentation. This condition
may not always hold if the IPv6 fragments also incur IPv4 encapsulation
and fragmentation over paths that transit IPv4 links with small MTUs.
Even then, only the fragmented Integrity Block (i.e., and not the
entire parcel) may need to be pulled into the contiguous head of
a kernel receive buffer.A TCP Parcel is an IP Parcel that includes an IP header plus
extensions with a Parcel Payload option formed as shown in
with Parcel Payload Length encoding
a value no larger than 16,777,215 (2**24 - 1) octets. The IP
header plus extensions is then followed by a TCP header plus
options (20 or more octets) followed by an Integrity Block
with (J + 1) consecutive 2-octet Checksums. The Integrity Block
is then followed by (J + 1) consecutive segments, where the first
segment is (L-4) octets in length and uses the 4-octet sequence
number found in the TCP header, each intermediate segment is L
octets in length (including its own 4-octet Sequence Number header)
and the final segment is K octets in length (including its own
4-octet Sequence Number header). The value L is encoded in the
IP header {Total, Payload} Length field while the overall length
of the parcel is determined by the Parcel Payload Length M as
discussed above.The source prepares TCP Parcels in an alternative adaptation of
TCP jumbograms . The source calculates a checksum
of the TCP header plus IP pseudo-header only (see: ),
but with the TCP header Sequence Number field temporarily set to 0
during the calculation since the true sequence number will be included
as an integrity check pseudo header for the first segment. The source then
writes the exact calculated value in the TCP header Checksum field (i.e.,
without converting calculated '0' values to 'ffff') and finally re-writes
the actual sequence number back into the Sequence Number field. (Nodes
that verify the header checksum first perform the same operation of
temporarily setting the Sequence Number field to 0 and then resetting
to the actual value following checksum verification.)The source then calculates the checksum of the first segment
beginning with the sequence number found in the full TCP header as a
4-octet pseudo-header then extending over the remaining (L-4) octet
length of the segment. The source next calculates the checksum for
each L octet intermediate segment independently over the length of
the segment (beginning with its sequence number), then finally
calculates the checksum of the K octet final segment (beginning
with its sequence number). As the source calculates each segment(i)
checksum (for i = 0 thru J), it writes the value into the
corresponding Integrity Block Checksum(i) field.Note: The parcel TCP header Source Port, Destination Port and
(per-segment) Sequence Number fields apply to each parcel segment,
while the TCP control bits and all other fields apply only to the
first segment (i.e., "segment(0)"). Therefore, only parcel segment(0)
may be associated with control bit settings while all other
segment(i)'s must be simple data segments.See for additional TCP considerations. See
for additional integrity considerations.A UDP Parcel is an IP Parcel that includes an IP header plus
extensions with a Parcel Payload option formed as shown in with Parcel Payload Length encoding a value
no larger than 16,777,215 (2**24 - 1) octets. The IP header plus
extensions is then followed by an 8-octet UDP header followed by
an Integrity Block with (J + 1) consecutive 2-octet Checksums
followed by (J + 1) transport layer segments. Each segment must
begin with a transport-specific start delimiter (e.g., a segment
identifier) included by the transport layer user of UDP. The
length of the first segment L is encoded in the IP {Total, Payload}
Length field while the overall length of the parcel is determined
by the Parcel Payload Length M as discussed above.The source prepares UDP Parcels in an alternative adaptation of
UDP jumbograms . The source first sets the UDP
header length field to 0, then calculates the checksum of the UDP header
plus IP pseudo-header (see: ) and writes the
exact calculated value into the UDP header Checksum field (i.e.,
without converting calculated '0' values to 'ffff').The source then calculates a separate checksum for each segment
for which checksums are enabled independently over the length of the
segment. As the source calculates each segment(i) checksum (for
i = 0 thru J), it writes the value into the corresponding Integrity
Block Checksum(i) field with calculated '0' values converted to
'ffff'; for segments with checksums disabled, the source instead
writes the value '0'.See: for additional integrity considerations.The IP parcel source unambiguously encodes the values L and M in
the corresponding header fields as specified above. The values J and
K are not encoded in header fields and must therefore be calculated
by intermediate and final destination nodes as follows:
64)
drop parcel;
if ((K = (T % (L + 2))) == 0) {
J--; K = L;
} else {
if ((J > 63) || ((K -= 2) <= 0))
drop parcel;
}
if ((TCP) && (J == 0) && ((K -= 4) <= 0))
drop parcel;]]>Note: from the above calculations, a minimal IP parcel is one
that sets L to at least 256 and includes at least one segment no
larger than L along with its corresponding 2-octet Checksum. In
addition, all IP parcels set L to at most 65535 and include at
most 64 segments along with their corresponding Checksums.When the network layer of the source assembles a {TCP,UDP}/IP
parcel it fully populates all IP header fields including the source
address, destination address and Parcel Payload option as discussed
above. The source also sets IP {Total, Payload} Length to L (between
256 and 65535) to distinguish the parcel from other jumbogram types
(see: ).The network layer of the source also maintains a randomly-initialized
4/8/12/16-octet (32/64/96/128-bit) (extended) Identification value for
each destination expressed in an Identification Extension Option for the
Internet Protocol to be included in the packet based on a new IP option
for IPv4 or an (Extended) Fragment Header for IPv6 (see: ). For each packet or parcel transmission,
the source sets the (extended) Identification to the current cached value
for this destination and increments the cached value by 1 (modulo
2**32/64/96/128) for each successive transmission. (The source can
then reset the cached value to a new random number, e.g., to maintain
an unpredictable profile.)The network layer of the source finally presents the parcel to
an interface for transmission to the next hop. For ordinary interface
attachments to parcel-capable links, the source simply admits each
parcel into the interface the same as for any IP packet where it
may be forwarded by one or more routers over additional consecutive
parcel-capable links possibly even traversing the entire forward
path to the final destination. If any node in the path does not
recognize the parcel construct, it drops the parcel and may return
an ICMP Parameter Problem message.When the next hop link does not support parcels at all, or when
the next hop link is parcel-capable but configures an MTU that is
too small to pass the entire parcel, the source breaks the parcel
up into individual IP packets (in the first case) or into smaller
sub-parcels (in the second case). In the first case, the source
can apply packetization using Generic Segment Offload (GSO), and
the final destination can apply restoration using Generic Receive
Offload (GRO) to deliver the largest possible parcel buffer(s)
to the transport layer. In the second case, the source can apply
parcellation to break the parcel into sub-parcels with each
containing the same (extended) Identification value and with the
S flag set appropriately. The final destination can then apply
reunification to deliver the largest possible parcel buffer(s) to
the transport layer. In all other ways, the source processes of
breaking a parcel up into individual IP packets or smaller sub-parcels
entail the same considerations as for a router on the path that
invokes these processes as discussed in the following subsections.Parcel probes that test the forward path's ability to pass parcels
set the Path MTU (PMTU field) to a non-zero value as discussed in . Each router in the path then rewrites PMTU in
a similar fashion as for .
Specifically, each router compares the parcel PMTU value with
the next hop link MTU in the parcel path and MUST (re)set PMTU to
the minimum value. The fact that the parcel transited a previous hop
link provides sufficient evidence of forward progress (since parcel
path MTU determination is unidirectional in the forward path only),
but nodes can also include the previous hop link MTU in their minimum
PMTU calculations in case the link may have an ingress size
restriction (such as a receive buffer limitation). Each parcel
also includes one or more transport layer segments corresponding
to the 5-tuple for the flow, which may include {TCP,UDP} segment
size probes used for packetization layer path MTU discovery . (See: for further details on parcel path probing.)When a router receives an IPv4 parcel it first compares Code with
255 and Check with the IPv4 header TTL; if either value differs, the
router drops the parcel and returns a negative Jumbo Report (see:
) subject to rate limiting. For all other IP parcels,
the router next compares the value L with the next hop link MTU. If the
next hop link is parcel capable but configures an MTU too small to admit
a parcel with a single segment of length L the router returns a positive
Jumbo Report (subject to rate limiting) with MTU set to the next hop link
MTU. If the next hop link is not parcel capable and configures an MTU
too small to pass an individual IP packet with a single segment of
length L the router instead returns a positive Parcel Report (subject
to rate limiting) with MTU set to the next hop link MTU. For IPv4
parcels, if the next hop link is parcel capable the router MUST
reset Check to the same value that would appear in the IPv4
header TTL field upon transmission to the next hop.If the router recognizes parcels but the next hop link in the path
does not, or if the entire parcel would exceed the next hop link MTU, the
router instead opens the parcel. The router then forwards each enclosed
segment in individual IP packets or in a set of smaller sub-parcels that
each contain a subset of the original parcel's segments. If the next
hop link is via an OMNI interface, the router instead follows OMNI
Adaptation Layer procedures. These considerations are discussed in
detail in the following sections.For transmission of individual IP packets over links that do not
support parcels, the source or router (i.e., the node) engages GSO
to perform packetization. The node first determines whether an
individual packet with segment of length L can fit within the next
hop link/path MTU. If an individual packet would be too large (and
if source fragmentation is not an option), the node drops the parcel
and returns a positive Parcel Report message (subject to rate limiting)
with MTU set to the next hop link/path MTU and with the leading portion
of the parcel beginning with the IP header as the "packet in error".
If an individual packet can be accommodated, the node removes the
Parcel Payload option, sets aside and remembers the Integrity Block
(and for TCP also sets aside and remembers the Sequence Number header
values of each non-first segment) then copies the {TCP,UDP}/IP headers
(but with the Parcel Payload option removed) followed by segment(i)
(for i= 0 thru J) into 'i' individual IP packets ("packet(i)").For each packet(i), the node then clears the TCP control bits
in all but packet(0), and includes only those TCP options that are
permitted to appear in data segments in all but packet(0) which may
also include control segment options (see:
for further discussion). The node then sets IP {Total, Payload}
Length for each packet(i) based on the length of segment(i) according
to the IP protocol standards .For each IPv6 packet(i), the node includes an Augmented IPv6
(Extended) Fragment Header that replaces the "Reserved" octet
with a "Parcel Index" octet as shown in .
The node then sets the (extended) Identification field to the
value found in the parcel header and writes the value 'i' in
the Index field. The node finally sets the "(P)arcel" bit to 1,
and sets the "More (S)egments" bit to 1 for each non-final segment
or 0 for the final segment.
For each IPv4 packet(i), the node instead includes an Identification
Extension Option with Parcel Index extension octet as specified in
. The node then sets
the Parcel Index octet values the same as for IPv6 above, sets the
(extended) Identification field to the value found in the parcel
header and sets the (D)ont Fragment flag to '1'.For each TCP/IP packet, the node then calculates the checksum
up to the end of packet(0)'s TCP/IP headers only according to but with the sequence number value saved and the
field set to 0. The node then adds Integrity Block Checksum(0) to the
calculated value and writes the sum into packet(0)'s TCP Checksum
field. The node then resets the Sequence Number field to packet(0)'s
saved sequence number and forwards packet(0) to the next hop. The
node next calculates the checksum of packet(1)'s TCP/IP headers with
the Sequence Number field set to 0 and saves the calculated value.
In each non-first packet(i) (for i = 1 thru J), the node then adds
the saved value to Integrity Block Checksum(i), writes the sum into
packet(i)'s TCP Checksum field, sets the TCP Sequence Number field
to packet(i)'s sequence number then finally includes the remainder
of segment(i) as the body of the packet while setting the IP length
field accordingly.For each UDP/IP packet, the node instead sets the UDP length field
according to in each packet(i) (for i= 0
thru J). If Integrity Block Checksum(i) is 0, the node then sets the
UDP Checksum field to 0, forwards packet(i) to the next hop and
continues to the next. The node otherwise next calculates the checksum
over packet(i)'s UDP/IP headers only according to .
If Integrity Block Checksum(i) is not 'ffff', the node then adds the
value to the header checksum; otherwise, the node re-calculates the
checksum for segment(i). If the re-calculated segment(i) checksum value
is 'ffff' or '0' the node adds the value to the header checksum;
otherwise, it discards this (errored) packet and continues to the
next packet(i). The node finally writes the total checksum value
into the packet(i) UDP Checksum field (or writes 'ffff' if the
total was '0').For each IP packet(i), the node then sets both the Fragment
Offset field and (M)ore fragments flag to '0' (and for IPv4
also sets the DF flag to '0'). The node then performs source
fragmentation if necessary while using both the (extended)
Identification and Parcel Index fields to identify the fragments
of the same packet(i). The node finally forwards packet(i) or
all of its constituent fragments to the next hop.Note: Packets resulting from packetization may be too large
to transit the remaining path to the final destination, such that
a router may drop the packet(s) and possibly also return an
ordinary ICMP PTB message. Since these messages cannot be
authenticated or may be lost on the return path, the original
source should take care in setting a segment size larger than
the known path MTU unless as part of an active probing service.Note: For all {TCP,UDP} packet(i)'s, the node can optionally
re-calculate and verify the segment checksum before forwarding,
but this may introduce unacceptable delay and processing overhead.
The final destination is therefore responsible for verifying
integrity on its own behalf, since intermediate network nodes
often do not perform upper layer integrity checks.For transmission of smaller sub-parcels over parcel-capable links,
the source or router (i.e., the node) first determines whether a single
segment of length L can fit within the next hop link MTU if packaged as
a (singleton) sub-parcel. If a singleton sub-parcel would too large, the
node returns a positive Jumbo Report message (subject to rate limiting)
with MTU set to the next hop link MTU and containing the leading portion
of the parcel beginning with the IP header, then performs packetization
as discussed in . Otherwise, the node
instead employs network layer parcellation to break the original parcel
into smaller groups of segments that would fit within the path MTU by
determining the number of segments of length L that can fit into each
sub-parcel under the size constraints. For example, if the node
determines that each sub-parcel can contain 3 segments of length L,
it creates sub-parcels with the first containing Integrity Block
Checksums/Segments 0-2, the second containing 3-5, the third containing
6-8, etc., and with the final containing any remaining Checksums/Segments.If the original parcel's Parcel Payload option has S set to '0', the
node then sets S to '1' in all resulting sub-parcels except the last
(i.e., the one containing the final segment of length K, which may be
shorter than L) for which it sets S to '0'. If the original parcel has
S set to'1', the node instead sets S to '1' in all resulting sub-parcels
including the last. The node next sets the Index field to the value
'i' which is the ordinal number of the first segment included in each
sub-parcel. (In the above example, the first sub-parcel sets Index
to 0, the second sets Index to 3, the third sets Index to 6, etc.).
If another router further down the path toward the final destination
forwards the sub-parcel(s) over a link that configures a smaller MTU,
the router may break it into even smaller sub-parcels each with Index
set to the ordinal number of the first segment included.The node next appends identical {TCP,UDP}/IP headers (including the
Parcel Payload option and any other extensions) to each sub-parcel while
resetting Index, S, {Total, Payload} Length (L) and Parcel Payload Length
(M) in each as discussed above. For TCP, the node then clears the TCP
control bits in all but the first sub-parcel and includes only those
TCP options that are permitted to appear in data segments in all but
the first sub-parcel (which may also include control segment options).
For both TCP and UDP, the node then resets the {TCP,UDP} Checksum
according to ordinary parcel formation procedures (see above). The
node then sets the TCP Sequence Number field to the value that appears
in the first sub-parcel segment while removing the first segment's
Sequence Number header (if present). The node finally sets PMTU to
the next hop link MTU then forwards each (sub-)parcel over the
parcel-capable next hop link.For transmission of original parcels or sub-parcels over OMNI
interfaces, the node admits all parcels into the interface
unconditionally since the OMNI interface MTU is unrestricted. The
OMNI Adaptation Layer (OAL) of this First Hop Segment (FHS) OAL
source node then forwards the parcel to the next OAL hop which may
be either an intermediate node or a Last Hop Segment (LHS) OAL
destination. OMNI interface parcellation and reunification
procedures are specified in detail in the remainder of this
section, while parcel encapsulation and fragmentation procedures
are specified in .When the OAL source forwards a parcel (whether generated
by a local application or forwarded over a network path that
transited one or more parcel-capable links), it first assigns a
monotonically-incrementing (modulo 255) adaptation layer Parcel ID.
If the parcel is larger than the OAL maximum segment size of 65535
octets, the OAL source first employs parcellation to break the
parcel into sub-parcels the same as for the network layer procedures
discussed above. This includes re-setting the Index, P, S, {Total,
Payload} Length (L) and Parcel Payload Length (M) fields in each
sub-parcel the same as specified in .The OAL source next assigns a different monotonically-incrementing
adaptation layer (extended) Identification value for each sub-parcel
of the same Parcel ID then performs adaptation layer encapsulation and
fragmentation and finally forwards each fragment to the next OAL hop
toward the OAL destination as necessary. (During encapsulation, the
OAL source examines the Parcel Payload option S flag to determine the
setting for the adaptation layer fragment header S flag according to
the same rules specified in .)When the sub-parcels arrive at the OAL destination, it retains
them along with their Parcel IDs and (extended) Identifications for
a short time to support reunification with peer sub-parcels of the
same original (sub-)parcel identified by the 3-tuple information
corresponding to the OAL source. This reunification entails the
concatenation of Checksums/Segments included in sub-parcels with
the same Parcel ID and with (extended) Identification values within
255 of one another to create a larger sub-parcel possibly even as
large as the entire original parcel. The OAL destination concatenates
the segments and Integrity Block Checksums for each sub-parcel in ascending
(extended) Identification value order, while ensuring that any sub-parcel
with TCP control bits set appears as the first concatenated element in
a reunified larger parcel and any sub-parcel with S flag set to '0'
appears as the final concatenation. The OAL destination then sets
S to '0' in the reunified (sub-)parcel if and only if one of its
constituent elements also had S set to '0'; otherwise, it sets
S to '1'.The OAL destination then appends a common {TCP,UDP}/IP header
plus extensions to each reunified sub-parcel while resetting Index,
S, {Total, Payload} Length (L) and Parcel Payload Length (M) in the
corresponding header fields of each. For TCP, if any sub-parcel has
TCP control bits set the OAL destination regards it as sub-parcel(0)
and uses its TCP header as the header of the reunified (sub-)parcel
with the TCP options including the union of the TCP options of all
reunified sub-parcels. The OAL destination then resets the
{TCP,UDP}/IP header checksum. If the OAL destination is also the
final destination, it then delivers the sub-parcels to the network
layer which processes them according to the 5-tuple information
supplied by the original source. If the OAL destination is not the
final destination, it instead forwards each sub-parcel toward the
final destination the same as for an ordinary IP packet as
discussed above.Note: Adaptation layer parcellation over OMNI links occurs only
at the OAL source while adaptation layer reunification occurs only
at the OAL destination (intermediate OAL nodes do not engage in the
parcellation/reunification processes). The OAL destination should
retain sub-parcels in the reunification buffer only for a short
time (e.g., 1 second) or until all sub-parcels of the original
parcel have arrived. The OAL destination then delivers full and/or
incomplete reunifications to the network layer (in cases where
loss and/or delayed arrival interfere with full reunification). Note: OMNI interface parcellation and reunification is an OAL
process based on the adaptation layer 3-tuple and not the network layer
5-tuple. This is true even if the OAL has visibility into network
layer information since some sub-parcels of the same original parcel
may be forwarded over different network paths.Note: Some implementations may be unable to apply adaptation layer
reunification for sub-parcels that have already incurred fragmentation
and reassembly. In that case, the adaptation layer can either linearize
each sub-parcel before applying reunification or deliver incomplete
reunifications or even individual sub-parcels to upper layers.When the original source or a router on the path opens a parcel
and forwards its contents as individual IP packets, these packets
will arrive at the final destination which can reassembly each
packet if necessary then hold them in a restoration buffer for a
short time before restoring the original parcel using GRO. The 5-tuple
information plus the (extended) Identification and (Parcel) Index
values provide sufficient context for GRO restoration which practical
implementations have proven can provide a robust service at high data
rates.When the original source or a router on the path opens a parcel
and forwards its contents as smaller sub-parcels, these sub-parcels
will arrive at the final destination which can hold them in a
reunification buffer for a short time or until all sub-parcels
have arrived. The 5-tuple information plus the Index, S flag and
(extended) Identification values provide sufficient context for
reunification.In both the restoration and reunification cases, the final
destination concatenates segments according to ascending Index
numbers to preserve segment ordering even if a small degree of
reordering and/or loss may have occurred in the networked path.
When the final destination performs restoration/reunification on
TCP segments, it must include the one with any TCP flag bits set
as the first concatenation and with the TCP options including the
union of the TCP options of all concatenated packets or sub-parcels.
For both TCP and UDP, any packet or sub-parcel containing the final
segment must appear as a final concatenation.The final destination can then present the concatenated parcel
contents to the transport layer with segments arranged in (nearly)
the same order in which they were originally transmitted. Strict
ordering is not mandatory since each segment will include a transport
layer protocol specific start delimiter with positional coordinates.
However, the Index field includes an ordinal value that preserves
ordering since each sub-parcel or individual IP packet contains an
integral number of whole transport layer protocol segments.Note: Restoration and/or reunification buffer management is
based on a hold timer during which singleton packets or sub-parcels
are retained until all members of the same original parcel have
arrived. It is recommended that implementations set a short hold
timer (e.g., 1 second) and advance any restorations/reunifications
to upper layers when the hold timer expires even if incomplete.Note: Since loss and/or reordering may occur in the network,
the final destination may receive a packet or sub-parcel with S
set to '0' before all other elements of the same original parcel
have arrived. This condition does not represent an error, but in
some cases may cause the network layer to deliver sub-parcels that
are smaller than the original parcel to the transport layer. The
transport layer simply accepts any segments received from all
such deliveries and will request retransmission of any segments
that were lost and/or damaged.Note: Restoration and/or reunification buffer congestion may
indicate that the network layer cannot sustain the service(s) at
current arrival rates. The network layer should then begin to
deliver incomplete restorations/reunifications or even individual
segments to the receive queue (e.g., a socket buffer) instead of
waiting for all segments to arrive. The network layer can manage
restoration/reunification buffers, e.g., by maintaining buffer
occupancy high/low watermarks.Note: Some implementations may be unable to apply network layer
restoration/reunification for packets/sub-parcels that have
already incurred adaptation layer reassembly/reunification. In that
case, the network layer can either linearize each packet/sub-parcel
before applying restoration/reunification or deliver incomplete
restorations/reunifications or even individual packets/sub-parcels
to upper layers.When a router or final destination returns a Parcel/Jumbo Report,
it prepares an ICMPv6 PTB message with Code
set to either Parcel Report or Jumbo Report (see: ) and with MTU set to either the
minimum MTU value for a positive report or to '0' for a negative
report. The node then writes its own IP address as the Parcel/Jumbo
Report source and writes the source address of the packet that
invoked the report as the Parcel/Jumbo Report destination (for
IPv4 Parcel Probes, the node writes the Parcel/Jumbo Report
address as an IPv4-Compatible IPv6 address ).
The node next copies as much of the leading portion of the invoking
packet as possible (beginning with the IP header) into the "packet
in error" field without causing the entire Parcel/Jumbo Report
(beginning with the IPv6 header) to exceed 256 octets in length. The
node then sets the Checksum field to 0 instead of calculating and
setting a true checksum since the UDP checksum (see below) already
provides an integrity check.Since IPv6 packets cannot transit IPv4 paths, and since middleboxes
often filter ICMPv6 messages as they transit IPv6 paths, the node next
wraps the Parcel/Jumbo Report in UDP/IP headers of the correct IP version
with the IP source and destination addresses copied from the Parcel/Jumbo
Report and with UDP port numbers set to the OMNI UDP port number . The node then calculates and sets
the UDP Checksum (and for IPv4 clears the DF bit). The node finally
sends the prepared Parcel/Jumbo Report to the original source of
the probe.Note: This implies that original sources that send IP parcels or
advanced jumbos must be capable of accepting and processing these
OMNI protocol UDP messages. A source that sends IP parcels or
advanced jumbos must therefore implement enough of the OMNI interface
to be able to recognize and process these messages.All parcels also serve as implicit probes and may cause either a
router in the path or the final destination to return an ordinary
ICMP error and/or
Packet Too Big (PTB) message concerning the parcel. A router in the
path or the final destination may also return a Parcel/Jumbo
Report (subject to rate limiting per )
as discussed in .To determine whether parcels can transit at least an initial
portion of the forward path toward the final destination, the
original source can also send IP parcels with a Parcel Payload
option PMTU field included and set to the next hop link MTU as
an explicit Parcel Probe. The Parcel Probe option format is shown
in , where option-length is set
to '12' for IPv4 and Opt Data Len is set to '8' for IPv6:
The parcel probe will cause the final destination
or a router on the path to return a Parcel/Jumbo Report or cause
the final destination to return an ordinary data packet with an
IP Jumbo Reply MTU option (see: ).A Parcel Probe can be included either in an ordinary data parcel
or a {TCP,UDP}/IP parcel with destination port set to '9' (discard)
. The probe will still contain a valid
{TCP,UDP} parcel header Checksum that any intermediate hops as
well as the final destination can use to detect mis-delivery,
while the final destination will process any parcel data in
probes with correct Checksums.If the original source receives a positive Parcel/Jumbo Report
or an ordinary data packet with an IP Jumbo Reply MTU option, it
marks the path as "parcels supported" and ignores any ordinary
ICMP and/or PTB messages concerning the probe. If the original
source instead receives a negative Jumbo Report or no report/reply,
it marks the path as "parcels not supported" and may regard any
ordinary ICMP and/or PTB messages concerning the probe (or its
contents) as indications of a possible path limitation.The original source can therefore send Parcel Probes in the
same IP parcels used to carry real data. The probes will transit
parcel-capable links joined by routers on the forward path possibly
extending all the way to the destination. If the original source
receives a positive Parcel/Jumbo Report or an ordinary data packet
with an IP Jumbo Reply MTU option, it can continue using IP parcels
after adjusting its segment size if necessary.The original source sends Parcel Probes unidirectionally in the
forward path toward the final destination to elicit a report/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 Parcel/Jumbo Reports and/or IP Jumbo
Reply MTU options must be packaged to reduce the risk of return path
filtering. For this reason, the Parcel Payload options included
in Parcel Probes and IP Jumbo Reply MTU options are always packaged
as IPv4 header or IPv6 Hop-by-Hop options while Parcel/Jumbo Reports
are returned as UDP/IP encapsulated ICMPv6 PTB messages with a
Parcel/Jumbo Report Code value (see:
).Original sources send ordinary parcels or discard parcels as
explicit Parcel Probes by setting the Parcel Payload PMTU to the
(non-zero) next hop link MTU. The source then sets Index, Parcel
Payload Length, and {Total, Payload} Length, then calculates the
header and per-segment checksums the same as for an ordinary
parcel. The source finally sends the Parcel Probe via the
outbound IP interface.Original sources can send Parcel Probes that include a large
segment size, but these may be dropped by a router on the path even
if the next hop link is parcel-capable. The original source may
then receive a Jumbo Report that contains only the MTU of the leading
portion of the path up to the router with the restrictive link. The
original source can instead send Parcel Probes with smaller segments
that would be likely to transit the entire forward path to the final
destination if all links are parcel-capable. For parcel-capable
paths, this may allow the original source to discover both the path
MTU and the MSS in a single message exchange instead of multiple.According to , IPv4 middleboxes (i.e.,
routers, security gateways, firewalls, etc.) that do not observe this
specification should drop IPv4 packets that contain option type
'11' (IPv4 Probe MTU) but some might instead either attempt to
implement or ignore the option altogether.
IPv4 middleboxes that observe this specification instead MUST process
the option as an implicit or explicit Parcel Probe as specified below.According to , IPv6 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) due to validation rules for basic jumbograms since
the parcel includes a non-zero IP {Total, Payload} Length. IPv6
middleboxes that observe this specification instead MUST process
the option as an implicit or explicit Parcel Probe as specified below.When a router that observes this specification receives an IPv4
Parcel Probe it first compares Code with 255 and Check with the IP
header TTL; if either value differs, the router drops the probe
and returns a negative Jumbo Report subject to rate
limiting. For all other IP Parcel Probes, if the next hop link is
non-parcel-capable the router compares PMTU with the next hop link
MTU and returns a positive Parcel Report subject to rate limiting
with MTU set to the minimum value. The router then applies
packetization to convert the probe into individual IP packet(s)
and forwards each packet to the next hop; otherwise, it drops
the probe.If the next hop link both supports parcels and configures an MTU
that is large enough to pass the probe, the router instead compares
the probe PMTU with the next hop link MTU. The router next MUST
(re)set PMTU to the minimum value then forward the probe to the
next hop (and for IPv4 first reset Check to the same value that
will appear in the IPv4 header TTL upon transmission to the next
hop). If the next hop link supports parcels but configures an MTU
that is too small to pass the probe, the router then applies
parcellation to break the probe into multiple smaller sub-parcels
that can transit the link. In the process, the router sets PMTU
to the minimum link MTU value in the first sub-parcel and sets
omits the PMTU field in all non-first sub-parcels (and for IPv4
resets Check in all sub-parcels). If the next hop link supports
parcels but configures an MTU that is too small to pass a
singleton sub-parcel of the probe, the router instead drops
the probe and returns a positive Jumbo Report subject to
rate limiting with MTU set to the next hop link MTU.The final destination may therefore receive one or more individual
IP packets or sub-parcels including an intact Parcel Probe. If the
final destination receives individual IP packets, it performs any
necessary integrity checks, applies restoration if possible
then delivers the (restored) parcel contents to the transport
layer. If the final destination receives an IPv4 Parcel Probe, it
first compares Code with 255 and Check with the IPv4 header TTL;
if either value differs, the final destination drops the probe and
returns a negative Jumbo Report. For all other Parcel Probes, if
the {TCP,UDP} port number is '9' (discard) the final destination
instead returns a positive Jumbo Report and discards the probe and
any of its associated sub-parcels without applying reunification.If the final destination receives a Parcel Probe (plus any of its
associated sub-parcels) for any other {TCP,UDP} port number, it
applies reunification and delivers the (reunified) parcel contents
to the transport layer. The destination then arranges to include
an IP Jumbo Reply MTU option in a return data packet/parcel
associated with the flow according to the format shown in :
For IPv4, the destination set option-type to '12' and
option-length to '16'/'20'/'24'/'28' according to the length
of the (extended) Identification field. The destination then sets
Rtn-PMTU to the minimum of 65535 and the value that will appear
in the PMTU field.For IPv6, the destination sets Option Type to '30' (hexadecimal)
and Opt Data Len to '12'/'16'/'20' according to the length of
the (extended) Identification field. The destination then sets
Min-PMTU to the minimum of 65535 and the outgoing link MTU and
sets Rtn-PMTU to the minimum of 65534 and the value that will
appear in PMTU (with the final bit cleared).For both IP protocol versions, the destination finally sets the
Path MTU and (extended) Identification fields to the values received
in the Parcel Probe, then sets other unused fields to 0. Note that
the option lengths differentiate the options from the shorter forms
of the same Option Types that appear in
and .After sending Parcel Probes (or ordinary parcels) the original
source may therefore receive UDP/IP encapsulated Parcel/Jumbo Reports,
ordinary data packets with IP Jumbo Reply MTU options, and/or transport
layer protocol probe replies. If the source receives a Parcel/Jumbo
Report, it verifies the UDP Checksum then verifies that the ICMPv6
Checksum is 0. If both Checksum values are correct, the node then
matches the enclosed PTB message with an original probe/parcel by
examining the ICMPv6 "packet in error" containing the leading
portion of the invoking packet. If the "packet in error" does not
match one of its previous packets, the source discards the
Parcel/Jumbo Report; otherwise, it continues to process.If the source receives a negative Parcel/Jumbo Report (i.e.,
one with MTU set to '0'), it marks the path as "parcels not supported".
Otherwise, the source marks the path as "parcels supported" and also
records the MTU value as the parcel path MTU (i.e., the portion of
the path up to and including the node that returned the Parcel/Jumbo
Report). If the MTU value is 65535 (plus headers) or larger, the MTU
determines the largest whole parcel that can transit the path without
packetization/parcellation while using any segment size up to and
including the maximum. For Reports that include a smaller MTU, the
value represents both the largest whole parcel size and a maximum
segment size limitation. In that case, the maximum parcel size that
can transit the initial portion of the path may be larger than the
maximum segment size that can continue to transit the remaining path
to the final destination.If the source receives an ordinary data packet for the flow that
includes an IP Jumbo Reply MTU option, it examines the (extended)
Identification to ensure that the reply matches one of the Parcel
Probes it previously sent for this same data flow. It then records
the PMTU value as the parcel/jumbo path MTU for this flow and marks
the path as "parcels and jumbos supported".For further discussion on parcel/jumbo probing alternatives, see:
.The {TCP,UDP}/IP header plus each segment of a (multi-segment) IP
parcel includes its own integrity check. This means that IP parcels
offer stronger and more discrete integrity checks for the same amount
of transport layer protocol data compared to an individual IP packet or
jumbogram. The {TCP,UDP} Checksum header integrity check can be verified
at each hop to ensure that parcels with errored headers are detected.
The per-segment Integrity Block Checksums are set by the source and
verified by the final destination, noting that TCP parcels must honor
the sequence number discipline discussed in .IP parcels can range in length from as small as only the {TCP,UDP}/IP
headers plus a single Integrity Block Checksum with a single segment to
as large as the headers plus (256 * 65535) octets. Although link layer
integrity checks such as CRC-32 provide sufficient protection for
contiguous data blocks up to approximately 9KB, reliance on link-layer
integrity checks may be inadvisable for links with significantly larger
MTUs and may not be possible at all for links such as tunnels over IPv4
that invoke fragmentation. Moreover, the segment contents of a received
parcel may arrive in an incomplete and/or rearranged order with respect
to their original packaging.Each network layer forwarding hop as well as the final destination
should verify the {TCP,UDP}/IP Checksum at its layer, since an errored
header could result in mis-delivery. If a network layer protocol entity
on the path detects an incorrect {TCP,UDP}/IP Checksum it should discard
the entire IP parcel unless the header(s) can somehow first be repaired
by lower layers.To support the parcel header checksum calculation, the network
layer uses modified versions of the {TCP,UDP}/IPv4 pseudo-header
found in ,
or the {TCP,UDP}/IPv6 pseudo-header found in Section 8.1 of
. Note that while the contents of the
two IP protocol version-specific pseudo-headers beyond the address
fields are the same, the order in which the contents are arranged
differs and must be honored according to the specific IP protocol
version as shown in . This allows for maximum
reuse of widely deployed code while ensuring interoperability.where the following fields appear in both pseudo-headers:
Source Address is the 4-octet IPv4 or 16-octet IPv6 source
address of the prepared parcel.Destination Address is the 4-octet IPv4 or 16-octet IPv6
destination address of the prepared parcel.zero encodes the constant value '0'.Next Header is the IP protocol number corresponding to the
transport layer protocol, i.e., TCP or UDP.Segment Length is the value that appears in the IP
{Total, Payload} Length field of the prepared parcel.[Index, P, S] is the combined 1-octet field that appears
in the Parcel Payload Option.Parcel Payload Length is the 3-octet value that appears in
the Parcel Payload Option field of the same name.Transport layer protocol entities coordinate per-segment checksum
processing with the network layer using a control mechanism such as
a socket option. If the transport layer sets a SO_NO_CHECK(TX) socket
option, the transport layer is responsible for supplying per-segment
checksums on transmission and the network layer forwards the IP parcel
to the next hop without further processing; otherwise, the network
layer supplies the per-segment checksums before forwarding. If the
transport layer sets a SO_NO_CHECK(RX) socket option, the transport
layer is responsible for verifying per-segment checksums on reception
and the network layer delivers each received parcel body to the
transport layer without further processing; otherwise, the network
layer verifies the per-segment parcel checksums before delivering.When the transport layer protocol entity of the source delivers a
parcel body to the network layer, it presents the (J + 1) segments
in canonical order either as a concatenated data buffer or as a list
of per-segment data buffers with each non-first TCP segment preceded
by a 4-octet Sequence Number field. The transport layer also optionally
includes/omits an Integrity Block of (J + 1) 2-octet Checksum fields
as ancillary data as follows. If the SO_NO_CHECK(TX) socket option is
set, the transport layer protocol includes the ancillary data block
and either calculates/writes each segment checksum (and for UDP with
'0' values written as 'ffff') or writes the value '0' to disable specific
UDP segment Checksums. If the SO_NO_CHECK(TX) socket options is clear,
for UDP the transport layer instead includes the ancillary data block
and writes the value '0' to disable or any non-zero value to enable
checksums for specific UDP segments; the transport layer instead
omits the ancillary data block either for TCP or when no UDP
per-segment controls are necessary.When the network layer of the source accepts the parcel body from
the transport layer protocol entity, if the SO_NO_CHECK(TX) socket option
is set the network layer appends the ancillary data Integrity Block and
{TCP,UDP}/IP headers then forwards the parcel to the next hop without
further processing. If the SO_NO_CHECK(TX) socket option is clear, the
network layer instead appends an Integrity Block header, calculates
the checksum for each {TCP,UDP} segment (or each UDP segment with a
non-zero value in the corresponding ancillary data Integrity Block
Checksum field, if present) and writes the calculated value into the
corresponding Integrity Block header per-segment field. (For UDP, if
the ancillary data Integrity Block per-segment checksum is set to '0',
the network layer writes the value '0' into the Integrity Block header;
the network layer otherwise writes calculated '0' values as 'ffff'.)
The network layer finally appends the {TCP,UDP}/IP headers and
forwards the parcel to the next hop.If the SO_NO_CHECK(RX) socket option at the destination is set,
when the network layer reunifies a parcel from one or more sub-parcels
received from the source it first verifies the {TCP,UDP}/IP header
checksum, then delivers the parcel segments (and unmodified Integrity
Block as ancillary data) to the transport layer protocol entity which
is then responsible for per-segment checksum verification. When the
network layer restores a parcel from one or more individual (TCP,UDP)/IP
packets received from the source, it instead delivers the parcel segments
and an ancillary data Integrity Block with each per-segment checksum set
to the per-packet checksum minus the {TCP,UDP}/IP header checksum; the
transport layer protocol entity is then again responsible for
per-segment checksum verification.If the SO_NO_CHECK(RX) socket option at the destination is clear,
the network layer instead verifies the checksum for each TCP segment (or
each UDP segment with a non-zero value in the corresponding Integrity
Block Checksum field) and marks a corresponding flag for the segment
in an ancillary data Flag Block as either "correct" or "incorrect".
(For restoration, the checksum verification includes the {TCP,UDP}/IP
headers while for reunification the verification covers only the segment
body. For UDP, if the Checksum is '0' the network layer unconditionally
marks the segment as "correct".) The network layer then delivers both
the parcel body and Flag Block as ancillary data to the transport
layer which can then determine which segments have correct/incorrect
checksums.Note: The Integrity Block itself is intentionally omitted from the IP
Parcel {TCP,UDP} header checksum calculation. This permits destinations
to accept as many intact segments as possible from received parcels with
checksum block bit errors, whereas the entire parcel would need to be
discarded if the header checksum also covered the Integrity Block.Note: When the source sets SO_NO_CHECK(TX) and sends a parcel
buffer with Integrity Block ancillary data, and the network layer
performs immediate packetization instead of sending as a parcel, the
network layer re-adjusts the per-packet checksums the same as specified
in .This specification introduces an IP advanced jumbo service as an
alternative to basic IPv6 jumbograms that also includes a path probing
function based on the mechanisms specified in .
The function employs an Advanced Jumbo Option with the same option
type and length values as for the Parcel Payload option, but with
the 8-bit Parcel Index and 24-bit Parcel Payload Length fields
converted to a 32-bit Jumbo Payload Length field as shown in
:
The source prepares an advanced jumbo by
first setting the IP {Total, Payload} Length field to the
special Jumbo Type value '1' to distinguish this from a basic
jumbogram or parcel. The source can begin by sending a
Jumbo Probe to pre-qualify the path for advanced jumbos
if necessary.To prepare a Jumbo Probe that will trigger a Jumbo Report,
the source can set {Protocol, Next Header} to {TCP,UDP}, set
the {TCP,UDP} port to '9' (discard) and either include no octets
beyond the {TCP,UDP} header or a single discard segment of the
desired probe size immediately following the header and with no
Integrity Block included. (The source can instead set the
{TCP,UDP} port to the port number for a current data flow in
order to receive IP Jumbo Reply MTU options in return packets
as discussed in .) The source then sets
Jumbo Payload Length to the length of the {TCP,UDP} header plus
the length of the discard segment plus the length of the full
IP header for IPv4 or the extension headers for IPv6.The source next sets the (extended) Identification the same
as for an IP Parcel Probe and sets the Jumbo Probe PMTU to the
next hop link MTU; for IPv4, the source also sets Code to 255
and Check to the
next hop TTL. The source then calculates the {TCP,UDP} Checksum
based on the same pseudo header as for an ordinary parcel (see:
) but with the Parcel Index and Payload
Length fields replaced with a 32-bit Jumbo Payload Length field
and with the Segment Length replaced with the Jumbo Type value
'1'. The source then calculates the checksum over the pseudo
header then continues the calculation over the entire length
of the probe segment. The source then sends the Jumbo Probe
via the next hop link toward the final destination.At each IPv4 forwarding hop, the router examines Code and Check
then drops the probe and returns a negative Jumbo Report if either
value is incorrect. If both values are correct, and if the next hop
link is jumbo-capable, the router compares PMTU to the next hop link
MTU, resets PMTU to the minimum value (and for IPv4 sets Check to
the next hop TTL) then forwards the probe to the next hop. If the
next hop link is not jumbo-capable, the router instead drops the
probe and returns a negative Jumbo Report.If the Jumbo Probe encounters an OMNI link, the OAL source can
either drop the probe and return a negative Jumbo Report or forward
the probe further toward the OAL destination using adaptation layer
encapsulation. If the OAL source already knows the OAL path MTU
for this OAL destination, it can encapsulate and forward the Jumbo
Probe with PMTU set to the minimum of itself and the known value
(minus the adaptation layer header size), and without adding any
padding octets.If the OAL path MTU is unknown, the OAL source can instead
encapsulate the Jumbo Probe in an adaptation layer IPv6 header
with a Jumbo Payload option and with NULL padding octets added
beyond the end of the encapsulated Jumbo Probe to form an
adaptation layer jumbogram as large as the minimum of PMTU
and (2**24 - 1) octets (minus the adaptation layer header size)
as a form of "jumbo-in-jumbo" encapsulation.The OAL source then writes this size into the Jumbo Probe
PMTU field and forwards the newly-created adaptation layer
jumbogram toward the OAL destination. If the jumbogram somehow
transits the path, the OAL destination then removes the
adaptation layer encapsulation, discards the padding, then
forwards the Jumbo Probe onward toward the final destination
(with each hop reducing PMTU if necessary).When a router on the path forwards a Jumbo Probe, it drops
and returns a Jumbo Report if the next hop MTU is insufficient;
otherwise, it forwards to the next hop toward the final destination.
When the final destination receives the Jumbo Probe, it returns
a Jumbo Report with the PMTU set to the maximum-sized jumbo that
can transit the path.When the Jumbo Probe reaches the final destination, the
destination first examines the {TCP,UDP} port number. If the
port number is '9' (discard), the destination returns a Jumbo
Report UDP message; otherwise, the destination prepares an IP
Jumbo Reply MTU option to include on a data packet on the
return path to the original source. Detailed descriptions
for these processes are found in .After successfully probing the path, the original source can
begin sending regular advanced jumbos by setting the IP
{Total, Payload} Length field to the special Jumbo Type value
'1', setting PMTU to 0, then calculating the Checksum the same as
described for probes above. When the final destination receives
an advanced jumbo, it first verifies the Checksum then delivers
the data to the transport layer without returning a Jumbo Report.
The source can continue to send advanced jumbos into the path
with the possibility that the path may change. In that case, a
router in the network may return an ICMP error, an ICMPv6 PTB,
or a Jumbo Report if the path MTU decreases.Note: If the OAL source can in some way determine that a very
large packet is likely to transit the OAL path, it can encapsulate
a Jumbo Probe to form an adaptation layer jumbogram even larger
than (2**24 - 1) octets with the understanding that the time
required to transit the path determines acceptable jumbogram
sizes.Note: The Jumbo Report message types returned in response to
both Parcel and Jumbo Probes are one and the same, and signify
that both parcels and advanced jumbos at least as large as the
reported MTU can transit the path.Common widely-deployed implementations include services such as TCP
Segmentation Offload (TSO) and Generic Segmentation/Receive Offload
(GSO/GRO). These services support a robust service that has been
shown to improve performance in many instances.UDP/IPv4 parcels have been implemented in the linux-5.10.67 kernel and
ION-DTN ion-open-source-4.1.0 source distributions. Patch distribution
found at: "https://github.com/fltemplin/ip-parcels.git".Performance analysis with a single-threaded receiver has shown that
including increasing numbers of segments in a single parcel produces
measurable performance gains over fewer numbers of segments due to more
efficient packaging and reduced system calls/interrupts. For example,
sending parcels with 30 2000-octet segments shows a 48% performance
increase in comparison with ordinary IP packets with a single
2000-octet segment.Since performance is strongly bounded by single-segment receiver
processing time (with larger segments producing dramatic performance
increases), it is expected that parcels with increasing numbers of
segments will provide a performance multiplier on multi-threaded
receivers in parallel processing environments.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].The IANA is instructed to create and maintain a new registry entitled
"IP Jumbogram Types". For IP packets that include a Jumbo Payload Option,
the IP {Total, Header} Length field encodes a "Jumbo Type" value instead
of an ordinary length. Initial values are given below:
In the control plane, original sources match any identifying
information in received Parcel/Jumbo Reports and IP Jumbo Reply MTU
options with their corresponding probes. If the information matches,
the report is likely authentic. In environments where stronger
authentication is necessary, nodes that send Parcel and/or Jumbo
Reports can apply the message authentication services specified
for AERO/OMNI.In the data plane, multi-layer security solutions may be needed
to ensure confidentiality, integrity and availability. Since parcels
and advanced jumbos are defined only for TCP and UDP, IPsec-AH/ESP
cannot be applied in transport mode although
they can certainly be used in tunnel mode at lower layers such as for
transmission of parcels and advanced jumbos over OMNI link secured
spanning trees, VPNs, etc. Since the network layer does not manipulate
transport layer segments, parcels and advanced jumbos do not interfere
with transport or higher-layer security services such as (D)TLS/SSL
which may provide greater flexibility in
some environments.IPv4 fragment reassembly is known to be dangerous at high data
rates where undetected reassembly buffer corruptions can result
from fragment misassociations . IPv6 is
less subject to these concerns when the 32-bit Identification field
is managed responsibly. However, both IPv4 and IPv6 can robustly
support high data rate reassembly using Identification Extension
Options for the Internet Protocol .Further security considerations related to IP parcels are found
in the AERO/OMNI specifications.This work was inspired by ongoing AERO/OMNI/DTN investigations. The
concepts were further motivated through discussions with colleagues.A considerable body of work over recent years has produced useful
segmentation offload facilities available in widely-deployed
implementations.With the advent of networked storage, big data, streaming media
and other high data rate uses the early days of Internetworking have
evolved to accommodate the need for improved performance. The need
fostered a concerted effort in the industry to pursue performance
optimizations at all layers that continues in the modern era. All
who supported and continue to support advances in Internetworking
performance are acknowledged.The following individuals are acknowledged for their contributions:
Scott Burleigh, Madhuri Madhava Badgandi, Bhargava Raman Sai Prakash.Accelerating UDP packet transmission for QUIC,
https://blog.cloudflare.com/accelerating-udp-packet-transmission-for-quic/BIG TCP, Netdev 0x15 Conference (virtual),
https://netdevconf.info/0x15/session.html?BIG-TCPTCP Extensions for High Performance are specified in , which updates earlier work that began in the late
1980's and early 1990's. These efforts determined that the TCP 16-bit
Window was too small to accommodate sustained transmission at high
data rates and devised a TCP Window Scale option to allow window
sizes up to 2^30. The work also defined a Timestamp option used
for round-trip time measurements and as a Protection Against Wrapped
Sequences (PAWS) at high data rates. TCP users of IP parcels are
strongly encouraged to adopt these measures.Since TCP/IP parcels only include control bits for the first
segment ("segment(0)"), nodes must regard all other segments of the
same parcel as data segments. When a node breaks a TCP/IP parcel out
into individual packets or sub-parcels, only the first packet/sub-parcel
contains the original segment(0) and therefore only its TCP header
retains the control bit settings from the original parcel TCP header.
If the original TCP header included TCP options such as Maximum Segment
Size (MSS), Window Scale (WS) and/or Timestamp, the node copies those
same options into the options section of the new TCP header.For all other packets/sub-parcels, the note sets all TCP header
control bits to '0' as data segment(s). Then, if the original parcel
contained a Timestamp option, the node copies the Timestamp option
into the options section of the new TCP header. Appendix A of
provides implementation guidelines for
the Timestamp option layout.Appendix A of also discusses Interactions
with the TCP Urgent Pointer as follows: "if the Urgent Pointer
points beyond the end of the TCP data in the current segment, then
the user will remain in urgent mode until the next TCP segment arrives.
That segment will update the Urgent Pointer to a new offset, and the
user will never have left urgent mode". In the case of IP parcels,
however, it will often be the case that the next TCP segment is
included in the same (sub-)parcel as the segment that contained
the urgent pointer such that the urgent pointer can be updated
immediately.Finally, if the parcel contains more than 65535 octets of data
(i.e., spread across multiple segments), then the Urgent Pointer
can be regarded in the same manner as for jumbograms as described
in Section 5.2 of .For each parcel, the transport layer can specify any L value
between 256 and 65535 octets. Transport protocols that send
isolated control and/or data segments smaller than 256 octets
should package them as ordinary packets or as the final segment
of a parcel. It is also important to note that segments smaller
than 256 octets are likely to include control information for
which timely delivery rather than bulk packaging is desired.
Transport protocol streams therefore often include a mix of
(larger) parcels and (smaller) ordinary packets.The transport layer should also specify an L value no larger
than can accommodate the maximum-sized transport and network layer
headers that the source will include without causing a single
segment plus headers to exceed 65535 octets. For example, if the
source will include a 28 octet TCP header plus a 40 octet IPv6
header with 24 extension header octets (plus a 2 octet per-segment
checksum) the transport should specify an L value no larger
than (65535 - 28 - 40 - 24 - 2) = 65441 octets.The transport can specify still larger "extreme" L values up
to 65535 octets, but the resulting parcels might be lost along
some paths with unpredictable results. For example, a parcel
with an extreme L value set as large as 65535 might be able to
transit paths that can pass jumbograms natively but might not
be able to transit a path that includes non-jumbo links. The
transport layer should therefore carefully consider the benefits
of constructing parcels with extreme L values larger than the
recommended maximum due to high risk of loss compared with only
minor potential performance benefits.Parcels that include extreme L values larger than the
recommended maximum and with a maximum number of included
segments could also cause a parcel to exceed 16,777,215
(2**24 - 1) octets in total length. Since the Parcel Payload
Length field is limited to 24 bits, however, the largest
possible parcel is also limited by this size. See also the
above risk/benefit analysis for parcels that include extreme
L values larger than the recommended maximum.After sending a Parcel/Jumbo Probe, the source may receive a
Parcel/Jumbo Report from either a router on the path or from the
final destination itself. Alternatively, the source can shape its
probes to request IP Jumbo Reply MTU options carried by ordinary
data packets on the return path from the destination.If a router or final destination receives a Parcel/Jumbo Probe but
does not recognize the parcel/jumbo constructs, it will likely drop the
probe without further processing and may return an ICMP error. The
original source will then consider the probe as lost, but may attempt
to probe again later, e.g., in case the path may have changed.When the source examines the "packet in error" portion of
a Parcel/Jumbo Report, it can easily match the Report against its
recent transmissions if the (extended) Identification value is
available. For "packets in error" that do not include an
(extended) Identification, the source can attempt to match
based on any other identifying information available; otherwise,
it should discard the message.If the source receives multiple Parcel/Jumbo Reports for a
single parcel/jumbo sent into a given path, it should prefer any
information reported by the final destination over information
reported by a router. For example, if a router returns a negative
report while the destination returns a positive report the latter
should be considered as more-authoritative. For this reason, the
source should provide a configuration knob allowing it to accept
or ignore reports that originate from routers, e.g., according
to the network trust model.When a destination returns a Parcel/Jumbo Report, it
can optionally pair the report with an ordinary data packet
that it returns to the original source. For example, the OMNI
specification includes a "super-packet" service that allows
multiple independent IP packets to be encapsulated as a
single adaptation layer packet. This is distinct from an
IP parcel in that each packet member of the super-packet
includes its own IP (and possibly other upper layer) header.A source can request to receive two different types of
parcel/jumbo path MTU feedback from the destination - a UDP
encapsulated Parcel/Jumbo Report in response to a probe
sent to port '9' (discard), or an ordinary data packet with
an IP Jumbo Reply MTU option in response to a probe
sent into an ordinary transport layer protocol flow. In some
environments, one or both of these MTU feedback types may be
erroneously dropped by a router along the return path. The
source may therefore attempt to probe first using "method A",
and then try again using "method B", e.g., if there is no response.
In environments where ongoing transport protocol sessions are
established, it is recommended that the source engage the
IP Jumbo Reply MTU option as "method A".Both historic and modern-day data links configure Maximum
Transmission Units (MTUs) that are far smaller than the desired
state for Internetworking futures. When the first Ethernet data
links were deployed many decades ago, their 1500 octet MTU set a
strong precedent that was widely adopted. This same size now
appears as the predominant MTU limit for most paths in the
Internet today, although modern link deployments with MTUs
as large as 9KB have begun to emerge.In the late 1980's, the Fiber Distributed Data Interface (FDDI)
standard defined a new link type with MTU slightly larger than 4500
octets. The goal of the larger MTU was to increase performance by a
factor of 10 over the ubiquitous 10Mbps and 1500-octet MTU Ethernet
technologies of the time. Many factors including a failure to harmonize
MTU diversity and an Ethernet performance increase to 100Mbps led to
poor FDDI market reception. In the next decade, the 1990's saw new
initiatives including ATM/AAL5 (9KB MTU) and HiPPI (64KB MTU) which
offered high-speed data link alternatives with larger MTUs but again
the inability to harmonize diversity derailed their momentum. By the
end of the 1990s and leading into the 2000's, evolution of the 1Gbps,
10Gbps, 100Gbps and even faster modern Ethernet data rates has
obscured the fact that 21st century Internetworks still operate
with 20th century MTUs!To bridge this gap, increased OMNI interface deployment in the
near future will provide a virtual link type that can pass IP parcels
over paths that transit legacy data links with small MTUs. Performance
analysis has proven that (single-threaded) receive-side performance is
bounded by transport layer protocol segment size, with performance
increasing in direct proportion with segment size. Experiments have
also shown measurable (single-threaded) performance increases by
including larger numbers of segments per parcel, with steady increases
for including increasing number of segments. However, parallel
receive-side processing will provide performance multiplier benefits
since the multiple segments that arrive in a single parcel can be
processed simultaneously instead of serially.In addition to the clear near-term benefits, IP parcels and
advanced jumbos will increase performance to new levels as future
links with very large MTUs in excess of 65535 octets begin to emerge.
With such large MTUs, the traditional CRC-32 (or even CRC-64) error
checking with errored packet discard discipline will no longer apply
for large parcels and advanced jumbos. Instead, packets larger than
a link-specific threshold will include Forward Error Correction (FEC)
codes so that errored packets can be repaired at the receiver's data
link layer then delivered to higher layers rather than being discarded
and triggering retransmission of large amounts of data. Even if the
FEC repairs are incomplete or imperfect, all parcels can still be
delivered to higher layers where the individual segment checksums
will detect and discard any damaged data not repaired by the link
and/or adaptation layers (advanced jumbos on the other hand would
require complete FEC repair).These new "super-links" will begin to appear mostly in the
network edges (e.g., high-performance data centers), however some
space-domain links that extend over enormous distances may also
benefit. For this reason, a common use case will include super-links
in the edge networks of both parties of an end-to-end session with an
OMNI link connecting the two over wide area Internetworks. Medium-
to moderately large-sized IP parcels over OMNI links will already
provide considerable performance benefits for wide-area end-to-end
communications while truly large parcels and advanced jumbos over
super-links can provide boundless increases for localized bulk
transfers in edge networks or for deep space long haul transmissions.
The ability to grow and adapt without practical bound enabled by IP
parcels and advanced jumbos will inevitably encourage new data link
development leading to future innovations in new markets that will
revolutionize the Internet.Until these new links begin to emerge, however, parcels will already
provide a tremendous benefit to end systems by allowing applications to
send and receive segment buffers as large as 65535 octets or even larger
in a single system call. By expanding the current operating system call
data copy limit from its current 16-bit length to a 32-bit length,
applications will be able to send and receive maximum-length parcel
buffers even if parcellation is needed to fit within the interface MTU.
For applications such as the Delay Tolerant Networking (DTN) Bundle
Protocol , this will allow transfer of entire
large protocol objects (such as DTN bundles) in a single system call.Continuing into the future, a natural progression beginning with
IP packets then moving to IP parcels should also lead to wide scale
adoption of advanced jumbos. Since advanced jumbos carry only a
single very large transport layer data segment, loss of even a
single jumbogram could invoke a major retransmission event. But, with
the advent of forward error correcting codes, future link types could
offer truly large MTUs. Advanced jumbos sent over such links would
then be equipped with an error correction "repair kit" that the link
far end can use to patch the jumbogram allowing it to be processed
further by upper layers. Delay Tolerant Networking (DTN) over
high-speed and long-delay optical links provides an example
environment suitable for such large packets.<< RFC Editor - remove prior to publication >>Changes from earlier versions:Submit for review.