IP ParcelsBoeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftIP packets (both IPv4 and IPv6) contain a single unit of upper layer
protocol data which becomes the retransmission unit in case of loss.
Upper layer protocols including the Transmission Control Protocol (TCP)
and transports over the User Datagram Protocol (UDP) prepare data units
known as "segments", with traditional arrangements including a single
segment per IP packet. This document presents a new construct known as
the "IP Parcel" which permits a single packet to carry multiple upper
layer protocol segments, essentially creating a "packet-of-packets". IP
parcels provide an essential building block 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 upper layer protocol data
which becomes the retransmission unit in case of loss. Upper layer
protocols such as the Transmission Control Protocol (TCP) and transports over the User Datagram Protocol (UDP)
(including QUIC , LTP
and others) prepare data units known as
"segments", with traditional arrangements including a single segment per
IP packet. This document presents a new construct known as the "IP
Parcel" which permits a single packet to carry multiple upper layer
protocol segments. This essentially creates a "packet-of-packets" with
the IP layer and full {TCP,UDP} headers appearing only once but with
possibly more than one segment included.Parcels are formed when an upper layer protocol entity identified
by the "5-tuple" (source address, destination address, source port,
destination port, protocol number) prepares a data buffer beginning with
an Integrity Block of up to 256 2-octet Checksums followed by their
corresponding upper layer protocol segments that can be broken out
into smaller sub-parcels and/or individual packets if necessary. All
segments except the final one must be equal in length and no larger
than 65535 octets (minus headers), while the final segment must not
be larger than the others but may be smaller. The upper layer protocol
entity then delivers the buffer, number of segments and non-final
segment size to lower layers which append a {TCP,UDP} header and
an IP header plus extensions that identify this as a parcel and
not an ordinary packet.Parcels can be forwarded over consecutive parcel-capable links in
a path until arriving at a router where the next hop is via a link
that does not support parcels, a parcel-capable link with a size
restriction, or an ingress middlebox Overlay Multilink Network
(OMNI) Interface that
spans intermediate Internetworks using adaptation layer encapsulation
and fragmentation. In the first case, the router breaks the parcel
into individual IP packets and forwards them via the next hop link.
In the second case, the router breaks the parcel into smaller
sub-parcels and forwards them via the next hop link. In the final
case, the OMNI interface breaks the parcel into smaller sub-parcels
if necessary then applies adaptation layer encapsulation and
fragmentation if necessary.These OMNI interface sub-parcels may then be recombined into one
or more larger parcels by an egress middlebox OMNI interface which
either delivers them locally or forwards them over additional
parcel-capable links on the path to the final destination. The
final destination can then further re-combine sub-parcels of the
same original parcel so as to present the largest possible data
unit to upper layers. Reordering and even loss or damage of
individual segments within the network is therefore possible, but
what matters is that the parcels delivered to the final destination
should be the largest practical size for best performance and that
loss or receipt of individual segments (and not parcel size)
determines the retransmission unit.The following sections discuss rationale for creating and shipping
IP parcels as well as the actual protocol constructs and procedures
involved. IP parcels provide an essential building block for improved
performance, efficiency and integrity while encouraging larger Maximum
Transmission Units (MTUs) in the Internet. It is further expected that
the parcel concept will inspire future innovation in applications,
operating systems, network equipment and data links.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 up to
256 upper layer protocol segments wrapped in an efficient package for
transmission and delivery (i.e., a "packet-of-packets") while a
"singleton IP parcel" is simply a parcel that contains a single segment.
IP parcels are distinguished from ordinary packets through the special
header constructions discussed in this document.The IP 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.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 upper layer header appears in each
parcel regardless of the number of segments included. This distinction
often provides a significant savings in overhead made possible only
by 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 special attention to byte ordering
"endianness" to ensure interoperability between diverse architectures.The Automatic Extended Route Optimization (AERO) and Overlay Multilink Network
Interface (OMNI) technologies
provide an ideal architectural framework for transmission of IP parcels.
AERO/OMNI are expected to provide an operational environment for IP
parcels beginning from the earliest deployment phases and extending to
accommodate continuous growth. As more and more parcel-capable links
begin to emerge, e.g., in data centers, edge networks, space-domain
links and other high data rate services, AERO/OMNI will provide
an essential transit service for true IP parcel Internetworking.The term "parcel-capable link" refers to any data link medium
(physical or virtual) capable of transiting a {TCP,UDP}/IP packet
that employs the parcel-specific constructions specified in this
document. The link MUST be capable of forwarding all parcels
with segment lengths no larger than the minimum of the link Maximum
Transmission Unit (MTU) and 65535, while first applying parcel
subdivision if necessary (see: ). Currently,
only the OMNI link satisfies these properties, but new and
existing link types are encouraged to incorporate parcel
support in their designs.The term "Maximum Transmission Unit (MTU)" is widely understood
in Internetworking terminology to mean the largest packet size that
can traverse a single link ("link MTU") or an entire path ("path MTU")
without requiring IP layer fragmentation. If the MTU value returned
during parcel path qualification is larger than 65535, it determines
the maximum parcel size with unrestricted segment size that a router
can forward over the path/link without requiring a router to perform
subdivision; otherwise, it determines both the maximum parcel and
segment sizes (see: ).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
(but non-standard) service similar in nature to the IP parcel
service described here, and its application has shown significant
performance increases due to the increased transfer unit size between
the operating system kernel and QUIC applications. Unlike IP parcels,
however, GSO/GRO perform 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.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 idea is 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 jumbos 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) message 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 should employ a maximum segment size no larger than 65535
octets (minus headers), while parcels that carry multiple segments may
themselves be significantly larger. Then, even if the network needs to
sub-divide the parcels into smaller sub-parcels for further forwarding
toward the final destination, an important performance optimization for
the original source, final destination and network path as a whole can
be realized. This performance advantage is accompanied by an overall
improvement in integrity and efficiency.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.An upper layer protocol entity (identified by the 5-tuple as
above) forms a parcel body when it prepares a data buffer containing
the concatenation of an Integrity Block of up to 256 2-octet Checksums
followed by their corresponding upper layer protocol segments (with
each TCP non-first segment preceded by a 4-octet Sequence Number).
All non-final segments MUST be equal in length while the final segment
MUST NOT be larger and MAY be smaller. Each non-final segment MUST NOT
be larger than the minimum of 65535 octets and the path MTU, minus the
length of the {TCP,UDP} header, minus the length of the IP header (plus
options/extensions), minus 2 octets for the per-segment Checksum.
(Note that this also satisfies the case of ingress middlebox OMNI
interfaces in the path that would process the headers as upper layer
protocol payload during IPv6 encapsulation/fragmentation.)The upper layer protocol entity then presents the buffer and
non-final segment size L to lower layers (noting that the buffer may be
larger than 65535 octets if it includes sufficient segments of a large
enough size to exceed that value). If the fist hop link is not parcel
capable, the lower layer prepares each segment from the buffer as an
independent IP packet as will be discussed further below. Otherwise,
if the buffer plus headers would together be no larger than the first
hop link MTU or path MTU, the lower layer then appends a single full
{TCP,UDP} header (plus options) followed by a single IP header (plus
options/extensions). If the buffer would cause a single parcel to
exceed the link/path MTU, the lower layer instead breaks the buffer
up into multiple smaller buffers (each with an integral number of
segments) and appends separate {TCP,UDP}/IP headers for each as
sub-parcels of the same original parcel.The IP layer then presents each (sub-)parcel to a network interface
attachment to either an ordinary parcel-capable link or an OMNI link
that performs adaptation layer encapsulation and fragmentation (see:
). The IP layer includes a special coding of
the Jumbo Payload option in the IPv4 or IPv6 header.For IPv4, the Jumbo Payload option format is similar
to except that the IP layer sets option
type to '00001011' and option length to '00010000' noting that the
length distinguishes this type from its obsoleted use as the "IPv4
Probe MTU" option . The option is formed
as shown in :The IP layer then sets Code to 255 and sets Check to the
same value that will appear in the TTL of the outgoing IPv4 header.
The IP layer next sets Nsegs to a value J between 0 and 255 and sets
Jumbo 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.
Next, the IP layer sets Identification as discussed in , sets the "(P)robe Path MTU" flag to '1' for probes
or '0' for non-probes and sets the "(S)ub-parcel" flag to '1' for
non-final sub-parcels or '0' for the final (sub-)parcel. The IP
layer finally sets the IPv4 header DF bit to 1 and Total Length
field to the non-final segment size L.For IPv6, the IP layer includes a Jumbo Payload option in an IPv6
Hop-by-Hop Options extension header formatted the same as for IPv4
above, but with option type set to '11001110', option length set
to '00001100' and with the Code/Check fields omitted. The option
is formed as shown in :The IP layer then sets Nsegs to a 1-octet value J
between 0 and 255 and sets the Jumbo Payload Length field 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. Next, the IP layer sets Identification as discussed
in , sets the P flag to '1' for probes or
'0' for non-probes and sets the S flag to '1' for non-final
sub-parcels or '0' for the final (sub-)parcel. The IP layer
finally sets the IPv6 header Payload Length field to L.The IP layer then prepares the rest of the {TCP,UDP}/IP parcel
according to the formats shown in :where the total number of segments is (J + 1), L
is the length of each non-final segment which MUST NOT be larger than
65535 octets (minus headers) and K is the length of the final segment
which MUST NOT be larger than L. (Note that when J is 0, K and L
are one and the same value.)The {TCP,UDP} header is then 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) upper layer
protocol 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 Numbers,
with the 4-octet length included in L and K.)Following parcel construction, the Nsegs value unambiguously
determines the number of 2-octet Checksums present in the Integrity
Block and (together with the IP {Total, Payload} length and Jumbo
Payload Length) also determines the number of parcel data segments
present. Receiving nodes that process IP parcels therefore observe
the following requirements:if the Jumbo Payload Length indicates insufficient space for
the full Integrity Block plus at least one data segment of
length K, the receiver discards the parcel.if the length of the payload following the Integrity Block
is (J * L) or less, the receiver processes all initial
Checksums along with their corresponding segments up to the
end of the payload and ignores any remaining Checksums.if the length of the payload following the Integrity Block is
greater than ((J + 1) * L) the receiver processes all Checksums
with their corresponding segments and ignores any remaining
payload beyond the end of the final segment.Note: 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 traverse slow IPv4 links with small
MTUs. In that case, performance is bounded by the unavoidable slow link
traversal and not the overhead for pulling a fragmented Integrity
Block 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 Jumbo Payload option formed as shown in
with Nsegs/J encoding one
less than the number of segments and Jumbo Payload length encoding
a value up to 16,777,215 (2**24 - 1). The IP header plus extensions
is then followed by a TCP header plus options (20 or more octets),
which is then 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 segment header) and the final segment is K octets in
length (including its own 4-octet Sequence Number segment header).
The value L is encoded in the IP header {Total, Payload} Length field
while J is encoded in the Nsegs octet. The overall length of the
parcel as well as final segment length K are determined by the
Jumbo Payload length M as discussed above.The source prepares TCP Parcels in a similar fashion as for simple
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 a pseudo header for the first segment. The source then writes the
calculated value in the TCP header Checksum field as-is (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 as-is.See: for further discussion.A UDP Parcel is an IP Parcel that includes an IP header plus
extensions with a Jumbo Payload option formed as shown in
with Nsegs/J encoding one less than
the number of segments and Jumbo Payload length encoding a value
up to 16,777,215 (2**24 - 1). 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)
upper layer protocol 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
J is encoded in the Nsegs octet. The overall length of the parcel
as well as the final segment length are determined by the Jumbo
Payload length M as discussed above.The source prepares UDP Parcels in a similar fashion as for simple
UDP jumbograms and therefore MUST set the UDP
header length field to 0. The source then calculates the checksum of
the UDP header plus IP pseudo-header (see: )
and writes the calculated value in the UDP header Checksum field as-is
(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 further discussion.The IP layer of the source maintains a randomly-initialized
32-bit cached Identification value for each destination. For
each parcel transmission, the IP layer sets the Identification
field in the Jumbo Payload to the current cached value for this
destination then increments the cached value by 1 (modulo 2**32).
The IP layer can subsequently reset each cached value to a new
random value at any time (e.g., to maintain an unpredictable
profile) noting that resetting too frequently may interfere
with opportunistic reassembly at destinations.The IP layer of the source next presents each parcel to a network
interface for transmission. For ordinary IP interface attachments to
parcel-capable links, the interface simply admits each parcel into
the link the same as for any IP packet after which it may then be
forwarded by any number of routers over additional consecutive
parcel-capable links possibly even traversing the entire forward
path to the final destination. If any router in the path does not
recognize the parcel construct, it may drop the parcel and return
an ICMP "Parameter Problem" message.When the underlying link is parcel-capable but configures an
MTU too small to transit the entire parcel, or when the underlying
link does not support parcels at all, the source breaks the parcel
up into smaller sub-parcels (in the first case) or into individual
IP packets (in the second case). For sub-parcels, each sub-parcel
will contain the same Identification value and with the S flag
set appropriately. This will allow the final destination to
reassemble in a way that allows it to deliver the largest possible
parcel buffers to its upper layer protocols. For individual IP
packets, no parcel framing is included but the process engages
Generic Segment Offload (GSO) and the final destination can
apply Generic Receive Offload (GRO) to recombine the packets into
a larger parcel before delivery to upper layers. In all other ways,
the source processes of breaking a larger parcel up into smaller
sub-parcels or individual IP packets entails the same considerations
as for a router on the path that invokes these processes as will
be discussed in the following subsections.Each parcel serves as an implicit probe that tests the forward
path's ability to pass parcels. Each parcel also includes a trailing
30-bit "Path MTU (PMTU)" field into which the source and each router
in the path writes the least significant 30 bits of the minimum link
MTU observed so far in a similar fashion as
and . (In particular, each
router compares the parcel PMTU value with the MTUs of both the
inbound and outbound links for the parcel and MUST (re)set PMTU to
the lower MTU. Each parcel also includes one or more upper layer
protocol segments corresponding to the 5-tuple for the flow, which
may also include {TCP,UDP} segment size probes used for packetization
layer path MTU discovery .
(See for further details on implicit/explicit
path probing.)When a router receives an IPv4 Parcel it first compares Code with
255 and Check with the IP header TTL/Hop Limit; if either value differs,
the router drops the parcel and return a negative Parcel Reply (see
). For all IP parcels, the router next compares
the value L with the next hop link MTU. If the next hop link MTU is
too small to pass either a singleton parcel or an individual IP packet
with segment of length L the router discards the parcel and returns a
positive Parcel Reply with MTU set to the next hop link MTU. For IPv4
parcels if the next hop link is parcel capable the router MUST then
reset Check to the same value that would appear in the TTL/Hop Limit
of the outgoing IP header for forwarding the parcel 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 singleton 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 proceeds according
to OMNI Adaptation Layer procedures. These considerations are discussed
in detail in the following sections.For transmission of singleton IP packets over links that do not
support parcels, the source or router (i.e., the node) engages GSO.
The node first determines whether a singleton parcel with segment of
length L can fit within the next-hop link MTU. If not, the node returns
a positive Parcel Reply message with MTU set to the next-hop link MTU
and containing the leading portion of the parcel beginning with the IP
header, then drops the parcel. Otherwise, the node removes the Jumbo
Payload option, sets aside and remembers the Integrity Block (and for
TCP also truncates the Sequence Number headers of each non-first segment
while remembering their values) then copies the {TCP,UDP}/IP headers
(but with the Jumbo Payload option removed) followed by segment(i)
(for i= 0 thru J) into individual singleton(i) IP packets. The node
then sets IP {Total, Payload} length for each singleton(i) based on
the length of segment(i) according to the standards .For each IPv6 singleton(i), the node includes an IPv6 Fragment Header
then sets the Identification field to the value found in the parcel header.
For each IPv4 singleton(i), the node sets the Identification field to the
least significant 16 bits of the value found in the parcel header and sets
the (D)ont Fragment flag to '1'. For each IP singleton(i), the node then
sets the Fragment Offset field to 0, sets the (M)ore fragments flag to '0',
then processes further according to upper layer protocol conventions as
follows.For TCP, the node clears the SYN/ACK flags in all except
singleton(0) then calculates the checksum for singleton(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
singleton(0)'s TCP checksum field. The node then resets the Sequence
Number field to singleton(0)'s saved sequence number and forwards
singleton(0) to the next hop. The node next calculates the checksum
of singleton(1)'s TCP/IP headers with the Sequence Number field set
to 0 and saves the calculated value. In each non-first singleton(i)
(for i = 1 thru J), the node then adds the saved value to Integrity
Block Checksum(i), writes the sum into singleton(i)'s TCP checksum
field, sets the TCP Sequence Number field to singleton(i)'s sequence
number then forwards singleton(i) to the next hop.For UDP, the node sets the UDP length field according to in each singleton(i) (for i= 0 thru J). If Integrity
Block Checksum(i) is 0, the node then sets the UDP header checksum
to 0, forwards singleton(i) to the next hop and continues to the next.
The node next calculates the checksum over singleton(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 continues
to the next singleton(i) (see note). The node finally writes the
total checksum value into the UDP checksum field for singleton(i)
(or writes 'ffff' if the total was '0') and forwards singleton(i)
to the next hop.Note: for each UDP singleton(i), the node must recalculate
the segment checksum if Checksum(i) is 'ffff', since that value is
shared by both '0' and 'ffff' calculated checksums. If recalculating
the checksum produces an incorrect value, segment(i) is considered
errored and the node can optionally drop or forward (noting that
the forwarded singleton would simply be discarded as an error by
the final destination).Note: for each {TCP,UDP} singleton(i), the node can optionally
re-calculate and verify the segment checksum unconditionally before
forwarding, but this may introduce undesirable extra delay and
processing overhead.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 not, the node returns a positive Parcel Reply
message with MTU set to the next-hop link MTU and containing the leading
portion of the parcel beginning with the IP header, then drops the parcel.
Otherwise, the node breaks 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 a 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
Checksums/Segments 3-5, etc., and with the final containing any remaining
Checksums/Segments.When the node breaks an original parcel into sub-parcels, it first
checks the "(S)ub-parcel" flag in the Jumbo Header. If the S flag is '0',
the node sets S to '1' in all resulting sub-parcels except the final
one (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 S flag is '1', the
node instead sets S to '1' in all resulting sub-parcels including the
final one. The node finally sets PMTU to the next hop link MTU.The node then appends identical {TCP,UDP}/IP headers (including the
Jumbo Payload option and any other extensions) to each sub-parcel while
resetting L and M in each according to the above equations with Nsegs/J
set to 2 for each intermediate sub-parcel and with Nsegs/J set to one
less than the remaining number of segments for the final sub-parcel. For
TCP, 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 Sequence Number field (if present) and also clears the SYN/ACK
flags in all sub-parcels except the first. For both TCP and UDP, the
node finally resets the {TCP,UDP} header checksum according to
ordinary parcel formation procedures (see above) then forwards each
(sub-)parcel over the outgoing parcel-capable link.Note: sub-dividing a larger parcel into two or more sub-parcels
entails replication of the {TCP,UDP}/IP headers (including the
Jumbo Payload option and any other extensions). For TCP, the process
entails copying the full TCP/IP header from the original parcel while
writing the sequence number of the first sub-parcel segment into the TCP
Sequence Number field, clearing the SYN/ACK flags if necessary as discussed
above and truncating the (new) first segment Sequence Number field. For
UDP, the process entails copying the full UDP/IP header from the original
parcel into each sub-parcel. For both TCP and UDP, the process finally
includes recalculating and resetting Nsegs and Jumbo Payload Length then
recalculating the {TCP,UDP} header checksum. Note that the per-segment
Integrity Block Checksum values in the sub-parcel segments themselves
are still valid and need not be recalculated.For transmission of original parcels or sub-parcels over OMNI
interfaces, all parcels are admitted into the OMNI 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 OAL intermediate node or a Last Hop
Segment (LHS) OAL destination. OMNI interface upper layer protocol processing
procedures are specified in detail in the remainder of this section, while
lower layer encapsulation and fragmentation procedures are specified in
detail in .When the OAL source forwards a parcel or sub-parcel (whether
generated by a local application or forwarded by other nodes over
one or more parcel-capable links), it first assigns a
monotonically-incrementing (modulo 255) "Parcel ID" for adaptation
layer processing. If the parcel is larger than the OAL maximum segment
size of 65535 octets, the OAL source then subdivides the parcel into
sub-parcels the same as for the IP layer procedures discussed above.
The OAL source next assigns a different monotonically-incrementing
adaptation layer 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 which forwards
them further toward the OAL destination as necessary. (During encapsulation,
the OAL source examines the Jumbo 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, the node can
optionally retain them along with their Parcel ID and Identifications
for a brief time to support re-combining with peer sub-parcels of the
same original parcel identified by the adaptation layer 4-tuple
(source, destination, Identification, Parcel ID). This re-combining
entails the concatenation of Checksums/Segments included in sub-parcels
with the same Parcel ID and with Identification values within 255 of
one another to create a larger sub-parcel possibly even as large as
the entire original parcel. Order of concatenation need not be strictly
enforced, with the exception that the sub-parcel with S flag set to '0'
must occur as a final concatenation and not as an intermediate. The
recombined (sub)parcel then sets the S flag to '0' if and only if
one of its recombined elements also had the S flag set to '0';
otherwise, it sets the S flag to '1'.The OAL destination then appends a common {TCP,UDP}/IP header plus
extensions to each re-combined sub-parcel while resetting J, K, L and M
in each according to the above equations. For TCP, if any sub-parcels
have the SYN/ACK flags set the OAL destination also sets
the SYN/ACK flags in the re-combined sub-parcel TCP header. The OAL
destination then resets the {TCP,UDP}/IP header checksum for each
re-combined sub-parcel. If the OAL destination is also the final
destination, it then delivers the sub-parcels to the IP layer which
processes them according to the 5-tuple information supplied by the
original source. Otherwise, the OAL destination forwards each sub-parcel
toward the final destination the same as for an ordinary IP packet as
discussed above.Note: sub-dividing of IP parcels over OMNI links occurs only at an
OAL ingress node while re-combining of IP parcels occurs only at an OAL
egress node. Therefore, intermediate OAL nodes do not participate in
the sub-dividing or recombining processes. For TCP, the SYN/ACK flags
must be managed as specified above to avoid confusing receivers with
gratuitous duplicate ACKs.Note: re-combining two or more sub-parcels into a larger parcel
entails a process in which the {TCP,UDP}/IP headers of non-first
sub-parcels are discarded and their included segments concatenated
following those of a first sub-parcel. For TCP, the process includes
setting the SYN/ACK flags in the TCP header only if SYN/ACK were set
in any of the original sub-parcels. For both TCP and UDP, the process
finally includes recalculating and resetting Nsegs and Jumbo Payload
Length then recalculating the {TCP,UDP} header checksum as discussed
above (the per-segment Integrity Block Checksums need not be
recalculated). The OAL destination can instead avoid this process
if it would negatively impact performance, noting that forwarding
individual sub-parcels without delay and without re-combining is
always acceptable.Note: sub-dividing and re-combining of IP parcels over OMNI links
occurs as an adaptation layer function based on the adaptation layer
4-tuple and not the network layer 5-tuple. The OAL must adhere to
this discipline even if 5-tuple information is available, since
some sub-parcels of the same original parcel may be forwarded
over different network paths.When a large parcel transits a path that includes links with
restrictive MTUs, the final destination may receive multiple
sub-parcels having the same 5-tuple and Identification value. The
final destination should hold the sub-parcels in a reassembly buffer
for a short time or until a sub-parcel with the S flag set to '0'
arrives. The final destination then concatenates the segments of
all non-final sub-parcels and finally concatenates the segments
of the final sub-parcel then passes the reassembled parcel to
upper layers.Due to the possibility of network loss and/or reordering, it will
often be the case that the final destination receives a sub-parcel
with S set to '0' before all other sub-parcels of the same original
parcel have arrived. This condition does not constitute an error,
but in some cases may cause the IP layer to deliver sub-parcels that
are smaller than the original parcel to upper layers. Upper Layers
simply process any segments received (i.e., regardless of the parcel
size), and will request retransmission of any segments that were
lost and/or damaged.If the original source or a router on the path opens a parcel
and forwards its contents as singleton IP packets, these packets
will arrive at the final destination which may collectively
reassemble them using GRO. The 5-tuple information plus the
IPv4 or IPv6 Identification fields populated by the original
source or router provide sufficient context for GRO reassembly,
which practical implementations have proven can provide a robust
reassembly capability at high data rates even for IPv4 with its
16-bit Identification limitation.Note: in both the sub-parcel and GRO reassembly cases, reassembly
entails concatenation of the segments in the order they were received
even though some small degree of reordering and/or loss may have
occurred in the networked path. This eliminates the need for a
reassembly offset value, since each sub-parcel or singleton IP
packet contains an integral number of whole upper layer protocol
segments which are not themselves fragmented. The IP layer can then
present the reassembled parcel contents to upper layers with segments
arranged in roughly the same order in which they were originally
transmitted, but strict ordering is not required since each segment
will include an upper layer protocol-specific start delimiter.Note: if the final destination's reassembly buffer holds sub-parcels
of "adjacent" parcels (i.e., those with identical 5-tuples, L values,
and with Identification values in close proximity) the destination can
optionally recombine sub-parcels of adjacent parcels to deliver to
upper layers. If so, however, the destination must avoid recombining
sub-parcels containing final segments of multiple original parcels.All parcels 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
an unsolicited "Parcel Reply" if the parcel cannot make further forward
progress.To unambiguously 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 the Jumbo Payload
option P flag set to '1' as an explicit "Parcel Probe". The probe
will elicit a Parcel Reply from a router or the final destination
(and possibly also an upper layer protocol-specific probe reply
from the final destination) while the parcel itself may continue
to make forward progress.If the original source receives a positive Parcel Reply, 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 Parcel Reply or no 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 send real data. The probes will traverse
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 Reply, it can continue using IP parcels
(while also adjusting its current segment size if necessary).The original source sends Parcel Probes unidirectionally in the
forward path toward the final destination to elicit a Parcel Reply,
since it will often be the case that IP parcels are supported only
in the forward path and not in the return path. Parcel Probes may be
dropped in the forward path by any node that does not recognize IP
parcels, but Parcel Replys must be packaged to avoid filtering since
parcels may not be recognized along portions of the return path. For
this reason, the Jumbo Payload options included in Parcel Probes
are always packaged as IPv4 header options or IPv6 Hop-by-Hop options
while Parcel Replys are returned as UDP/IP encapsulated ICMPv6 PTB
messages with a "Parcel Reply" Code value (see: ).Original sources send ordinary parcels as explicit Parcel Probes
by setting the Jumbo Payload P flag to '1' and PMTU to the least
significant 30 bits of the first hop link MTU. The source can also
form a NULL probe/parcel by setting Protocol to "No Next Header (59)"
and including an Integrity Block with one or more Checksum fields set
to '0' followed by a corresponding number of NULL segments with zero,
random and/or other disposable payloads. The source then sets {Nsegs,
Jumbo Payload Length, IPv4 Total Length} and 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.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
'00001011' ("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 the different option 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/Hop Limit; if either value differs, the router MUST drop the probe
and return a negative Parcel Reply (see below). For all other IP
Parcel Probes, if the next hop link is non-parcel-capable the router
compares the PMTU value with the MTU of the inbound and next hop link
MTUs for the probe and MUST (re)set PMTU to the lower value. The router
then MUST return a positive Parcel Reply (see below) and convert the
probe into singleton IP packet(s) the same as was described in . If the next hop IP link configures a sufficiently
large MTU to pass the packet(s), the router converts the probe and
MUST forward each singleton packet to the next hop; otherwise, it
drops the probe. If the next hop IP link both supports parcels and
configures an MTU that is large enough to pass the parcel, the router
instead compares the probe PMTU value with the MTUs of both the inbound
and next hop links for the probe and MUST (re)set PMTU to the lowest
value. The router then MUST forward the Parcel Probe to the next hop
(after resetting Check to the same value that will appear in the TTL
of the outgoing header for IPv4). If the next hop IP link supports
parcels but configures an MTU that is too small to pass the probe,
it resets PMTU (and Check if necessary) the same as above then
subdivides the probe into multiple smaller probes that can
traverse the link.The final destination may therefore receive either one or more
ordinary IP packets or intact Parcel Probes. If the final destination
receives ordinary IP packets, it performs any necessary integrity checks,
applies GRO if possible then delivers the packets or parcels to upper
layers which will return an upper layer probe response if necessary.
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 MUST drop the probe and return
a negative Parcel Reply. Otherwise, the final destination compares
the probe PMTU value with the MTU of the inbound link and MUST
reset PMTU to the lower MTU. The final destination then MUST return
a positive Parcel Reply and deliver the probe contents to upper
layers the same as for an ordinary IP parcel.When a router or final destination returns a Parcel Reply, it
prepares an ICMPv6 PTB message with Code set to
"Parcel Reply" (see: ) and with
MTU set to either the PMTU value reported in the probe/parcel for a positive
reply or to the value '0' for a negative reply. The node then writes its
own IP address as the Parcel Reply source and writes the source of the
Parcel Probe as the Parcel Reply destination (for IPv4 Parcel Probes,
the node writes the Parcel Reply address as an IPv4-Compatible IPv6
address ). The node next copies as much of
the leading portion of the probe/parcel (beginning with the IP header)
as possible into the "packet in error" field without causing the Parcel
Reply to exceed 512 octets in length, then calculates the ICMPv6 header
checksum. Since IPv6 packets cannot traverse IPv4 paths, and since
middleboxes often filter ICMPv6 messages as they traverse IPv6 paths,
the node next wraps the Parcel Reply in UDP/IP headers of the correct
IP version with the IP source and destination addresses copied from
the Parcel Reply and with UDP port numbers set to the UDP port number
for OMNI . In the process, the
node either calculates or omits the UDP checksum as appropriate and
(for IPv4) clears the DF bit. The node finally sends the prepared
Parcel Reply to the original source of the probe.After sending a Parcel Probe (or an ordinary parcel) the original
source may therefore receive a UDP/IP encapsulated Parcel Reply (see
above) and/or one or more upper layer protocol probe replies. If the
source receives a Parcel Reply, it first verifies the checksum then
matches the enclosed PTB message with the original probe/parcel by
examining the Identification field echoed in the ICMPv6 "packet in
error" field containing the leading portion of the probe. If PTB does
not match, the source discards the Parcel Reply; otherwise, it
continues to process. If the Parcel Reply MTU is '0', the source marks
the path as "parcels not supported"; otherwise, it marks the path as
"parcels supported" and also records the MTU value as the MTU for the
parcel path (i.e., the portion of the path up to and including the node
that returned the Parcel Reply). If the MTU value is 65535 or larger,
the MTU determines the largest whole parcel size that can traverse the
parcel path without subdivision while using any segment size up to and
including the maximum. If the MTU value is smaller than 65535, the MTU
represents both the largest whole parcel size and a maximum segment
size limitation. In both cases, the maximum segment size that can
traverse the parcel path may be larger than the maximum segment size
that can continue to traverse the remaining path to the final
destination, which can only be determined through upper layer
protocol probes (i.e., either as individual probe packets or
as payloads of the Parcel Probes).Note: The original source includes Code and Check fields as the
first 2 octets of both ordinary parcels and Parcel Probes in case a
router on the path overwrites the values in a wayward attempt to
implement . Parcel Probe recipients should
therefore regard a Code value other than 63 as an indication that
the field was either intentionally or accidentally altered by a
previous hop node.Note: If a router or final destination receives a Parcel Probe but
does not recognize the parcel construct, it drops 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.Note: On links that include a forward error correction capability,
in-transit damage to the Parcel Probe headers may be corrected as a
lower-layer function of the receiver before the headers are examined
by the network layer.The {TCP,UDP}/IP header plus each segment of a (multi-segment) IP
parcel includes its own integrity check. This means that IP parcels can
support stronger and more discrete integrity checks for the same amount
of upper layer protocol data compared to an ordinary IP packet or
Jumbogram. The {TCP/UDP} header integrity checks 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 non-zero length
segment to as large as the headers plus (256 * (65535 minus headers)) octets.
Although 32-bit link layer integrity checks 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.Lower layer protocol entities calculate and verify {TCP,UDP}/IP
parcel header Checksums at their layer, since an errored header could
result in mis-delivery to the wrong upper layer protocol entity. If a
lower layer protocol entity on the path detects an incorrect
{TCP,UDP}/IP Checksum it discards the entire IP parcel unless the
header(s) can somehow be repaired.To support the parcel header checksum calculation, lower layer
protocol entities use 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
but with different ordering: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 upper
layer protocol, i.e., TCP or UDP.Segment Length is the value that appears in the IPv4 Total
Length or IPv6 Payload Length field of the prepared parcel.Nsegs is a 1-octet value one less than the number of segments
included, and must contain a number between 0 and 255 (this is
the same value that appears in the Jumbo Payload Option Nsegs
field).Upper-Layer Packet Length is the 3-octet length of the
{TCP,UDP} header plus data (this value can be derived from
the Jumbo Payload Length by subtracting the IPv4 header length
for IPv4 or IPv6 extension header length for IPv6).Upper layer protocol entities use socket options to coordinate
per-segment checksum processing with lower layers. If the upper layer
sets a SO_NO_CHECK(TX) socket option, the upper layer is responsible for
supplying per-segment checksums on transmission and the lower layer
forwards the IP parcel to the next hop without further processing;
otherwise, the lower layer supplies the per-segment checksums before
forwarding. If the upper layer sets a SO_NO_CHECK(RX) socket option,
the upper layer is responsible for verifying per-segment checksums on
reception and the lower layer delivers each received parcel body to
the upper layer without further processing; otherwise, the lower
layer verifies the per-segment parcel checksums before delivering.When the upper layer protocol entity of the source sends a parcel
body to lower layers, it prepends an Integrity Block of (J + 1) 2-octet
Checksum fields and includes a 4-octet Sequence Number field with each
TCP non-first segment. If the SO_NO_CHECK(TX) socket option is set, the
upper layer protocol either calculates each segment checksum and writes
the value into the corresponding Checksum field (and for UDP with '0'
values written as 'ffff') or writes the value '0' to disable checksums
for specific UDP segments. If the SO_NO_CHECK(TX) socket options is
clear, for UDP the upper layer instead writes the value '0' to disable
or any non-zero value to enable checksums for specific segments (for
TCP, the upper layer instead writes any zero or non-zero value).When the lower layer protocol entity of the source receives the
parcel body from upper layers, if the SO_NO_CHECK(TX) socket option
is set the lower layer appends the {TCP,UDP}/IP headers and forwards
the parcel to the next hop without further processing. If the
SO_NO_CHECK(TX) socket option is clear, the lower layer instead
calculates the checksum for each TCP segment (or each UDP segment
with a non-zero value in the corresponding Integrity Block Checksum
field) and overwrites the calculated value into the Checksum field
(and for UDP with '0' values written as 'ffff').When the lower layer protocol entity of the destination receives a
parcel from the source, if the SO_NO_CHECK(RX) socket option is set the
lower layer delivers the parcel body to the upper layer without further
processing, and the upper layer is responsible for per-segment checksum
verification. If the SO_NO_CHECK(RX) socket option is clear, the lower
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 field for the segment in an
ancillary data structure as either "correct" or "incorrect". (For UDP,
if the Checksum is '0' the lower layer protocol unconditionally marks
the segment as "correct".) The lower layer then delivers both the parcel
body (beginning with the Integrity block) and ancillary data to the
upper 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: IP parcels that set {Protocol, Next Header} to
"No Next Header (59)" do not include a {TCP,UDP} Checksum field and
therefore do not include a header checksum. Intermediate nodes simply
forward these NULL parcels without verifying a header checksum,
while destination nodes simply discard them after returning a Parcel
Reply, if necessary.True IPv6 jumbograms are distinguished from IPv6 parcels by
including a zero IPv6 Payload Length and an IPv6 Hop-by-Hop
Option with type '11001110' and length '00000100'. The Jumbo
Payload option format and all aspects of IPv6 jumbogram processing
are exactly as specified in .True IPv4 jumbograms are distinguished from IPv4 parcels by
including a zero IPv4 Total Length and an IPv4 option with type
'00001011' and length '00000110'. The Jumbo Payload option format
and all aspects of IPv4 jumbogram processing are exactly the same
as for IPv6 jumbograms.This specification augments IP jumbograms by also providing a
Jumbo Path Qualification function using the mechanisms specified
in . The function employs a "Jumbo Probe"
formed exactly the same as for Parcel Probes, but with Nsegs/Jumbo
Payload Length set to '0' and with the P and S flags omitted and
PMTU expanded as a 32-bit field. The Jumbo Probe also sets the
IP {Total, Payload} length fields to '0', sets {Protocol,
Next Header} to "No Next Header (59)" and includes no octets
beyond the IP header. The purpose of the Jumbo Probe is to
determine whether the entire path from the source to the
destination is jumbo-capable (i.e., one in which all links
recognize jumbograms and configure an MTU larger than 65535
octets) as well as to determine the jumbo path MTU.The source sets the Jumbo Probe PMTU to the 32-bit MTU of
the (jumbo-capable) outgoing link, (and for IPv4 sets Code to
255 and sets Check to the next hop TTL/Hop Limit) then sends the
probe via the link toward the final destination. At each IPv4
forwarding hop, the router examines Code and Check and returns
a negative "Jumbo Reply" (i.e., prepared the same as a Parcel
Reply) if either value is incorrect. Otherwise, if the next hop
link MTU is jumbo-capable the router sets PMTU to the lower of
the current PMTU and incoming/outgoing link MTUs (and for IPv4
sets Check to the next hop TTL) then silently 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 Reply.If the Jumbo Probe encounters an OMNI link, the OAL source can
either drop the probe and return a negative Jumbo Reply or forward
the probe further toward the OAL destination using adaptation layer
encapsulation. In a first option, if the OAL source has a table
of known PMTUs for selected OAL destinations it can encapsulate
and forward the Jumbo Probe based on the known PMTU value. In a
second option, the OAL source can encapsulate the Jumbo Probe in
the adaptation layer IPv6 header with a jumbo payload option and
with (PMTU - headers) NULL padding octets added beyond the end of
the encapsulated Jumbo Probe to form an actual adaptation layer
probe. The OAL source then forwards the probe via the path toward
the OAL destination, where it may be lost due to a link restriction.
If the probe somehow traverses the path, the OAL destination then
removes the adaptation layer encapsulation, discards the trailing
padding, resets PMTU and Check and forwards the original
Jumbo Probe further toward the final destination.If the Jumbo Probe reaches the final destination, the final
destination returns a positive Jumbo Reply with the PMTU set to
the maximum-sized jumbogram that can transit the path. (Note that
the jumbo probing process is conducted independently of any parcel
probing, and that the two processes could very possibly yield
very different results.)Common widely-deployed implementations include services such as TCP
Segmentation Offload (TSO) and Generic Segmentation/Receive Offload
(GSO/GRO). These services support a robust (but non-standard) 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].In the control plane, original sources match the Identification
values in received Parcel Replys with their corresponding Parcels
or Parcel Probes. If the values match, the reply is likely authentic.
In environments where stronger authentication is necessary, nodes
that send Parcel Replys 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
are defined only for TCP and UDP, IP layer securing services such as
IPsec-AH/ESP cannot be applied directly to
parcels, although they can certainly be used at lower layers such as
for transmission of parcels over VPNs and/or OMNI link secured
spanning trees. Since the IP layer does not manipulate segments
exchanged with upper layers, parcels do not interfere with
transport- or higher-layer security services such as (D)TLS/SSL
which may provide greater flexibility in
some environments.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 on the IETF intarea
and 6man lists as well as with Boeing colleagues.A considerable body of work over recent years has produced useful
"segmentation offload" facilities available in widely-deployed
implementations.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-TCPBoth historic and modern-day data links configure Maximum Transmission
Units (MTUs) that are far smaller than the desired state for IP parcel
transmission 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, emergence of the 1Gbps,
10Gbps and even faster Ethernet performance levels seen today has
obscured the fact that the modern Internet of the 21st century is
still operating 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 traverse traditional data links with
small MTUs. Performance analysis has proven that (single-threaded)
receive-side performance is bounded by upper 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 will increase
performance to new levels as future parcel-capable links with very
large MTUs begin to emerge. These links will provide MTUs far in excess
of 64KB to as large as 16MB. 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. Instead, parcels larger than a
link-specific threshold will include Forward Error Correction (FEC)
codes so that errored parcels can be repaired at the receiver's data
link layer then delivered to upper 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 upper layers where the individual segment checksums
will detect and discard any damaged data not repaired by lower layers.These new "super-links" will appear mostly in the network edges
(e.g., high-performance data centers) and not as often in the middle
of the Internet. (However, some space-domain links that
extend over enormous distances may also benefit.) For this reason, a
common use case will include parcel-capable 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 IP parcels 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 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 larger than 65535 octets 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
lower layers need to break them into multiple parcels to fit within the
underlying interface MTU. For applications such as the Delay Tolerant
Networking (DTN) Bundle Protocol , this will
allow applications to send and receive entire large upper layer
protocol constructs (such as DTN bundles) in a single system call.<< RFC Editor - remove prior to publication >>Changes from earlier versions:Submit for Intarea Standards Track RFC Publication.