Internet DRAFT - draft-ietf-intarea-ipv4-id-update
draft-ietf-intarea-ipv4-id-update
Internet Area WG J. Touch
Internet Draft USC/ISI
Updates: 791,1122,2003 November 27, 2012
Intended status: Proposed Standard
Expires: May 2013
Updated Specification of the IPv4 ID Field
draft-ietf-intarea-ipv4-id-update-07.txt
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Abstract
The IPv4 Identification (ID) field enables fragmentation and
reassembly, and as currently specified is required to be unique
within the maximum lifetime for all datagrams with a given
source/destination/protocol tuple. If enforced, this uniqueness
requirement would limit all connections to 6.4 Mbps. Because
individual connections commonly exceed this speed, it is clear that
existing systems violate the current specification. This document
updates the specification of the IPv4 ID field in RFC791, RFC1122,
and RFC2003 to more closely reflect current practice and to more
closely match IPv6 so that the field's value is defined only when a
datagram is actually fragmented. It also discusses the impact of
these changes on how datagrams are used.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................3
3. The IPv4 ID Field..............................................4
3.1. Uses of the IPv4 ID Field.................................4
3.2. Background on IPv4 ID Reassembly Issues...................5
4. Updates to the IPv4 ID Specification...........................6
4.1. IPv4 ID Used Only for Fragmentation.......................7
4.2. Encourage Safe IPv4 ID Use................................8
4.3. IPv4 ID Requirements That Persist.........................8
5. Impact of Proposed Changes.....................................9
5.1. Impact on Legacy Internet Devices.........................9
5.2. Impact on Datagram Generation............................10
5.3. Impact on Middleboxes....................................11
5.3.1. Rewriting Middleboxes...............................11
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5.3.2. Filtering Middleboxes...............................12
5.4. Impact on Header Compression.............................12
5.5. Impact of Network Reordering and Loss....................13
5.5.1. Atomic Datagrams Experiencing Reordering or Loss....13
5.5.2. Non-atomic Datagrams Experiencing Reordering or Loss14
6. Updates to Existing Standards.................................14
6.1. Updates to RFC 791.......................................14
6.2. Updates to RFC 1122......................................15
6.3. Updates to RFC 2003......................................16
7. Security Considerations.......................................16
8. IANA Considerations...........................................17
9. References....................................................17
9.1. Normative References.....................................17
9.2. Informative References...................................17
10. Acknowledgments..............................................19
1. Introduction
In IPv4, the Identification (ID) field is a 16-bit value that is
unique for every datagram for a given source address, destination
address, and protocol, such that it does not repeat within the
maximum datagram lifetime (MDL) [RFC791][RFC1122]. As currently
specified, all datagrams between a source and destination of a given
protocol must have unique IPv4 ID values over a period of this MDL,
which is typically interpreted as two minutes, and is related to the
recommended reassembly timeout [RFC1122]. This uniqueness is
currently specified as for all datagrams, regardless of fragmentation
settings.
Uniqueness of the IPv4 ID is commonly violated by high speed devices;
if strictly enforced, it would limit the speed of a single protocol
between two IP endpoints to 6.4 Mbps for typical MTUs of 1500 bytes
[RFC4963]. It is common for a single connection to operate far in
excess of these rates, which strongly indicates that the uniqueness
of the IPv4 ID as specified is already moot. Further, some sources
have been generating non-varying IPv4 IDs for many years (e.g.,
cellphones), which resulted in support for such in ROHC [RFC5225].
This document updates the specification of the IPv4 ID field to more
closely reflect current practice, and to include considerations taken
into account during the specification of the similar field in IPv6.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
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In this document, the characters ">>" proceeding an indented line(s)
indicates a requirement using the key words listed above. This
convention aids reviewers in quickly identifying or finding this
document's explicit requirements.
3. The IPv4 ID Field
IP supports datagram fragmentation, where large datagrams are split
into smaller components to traverse links with limited maximum
transmission units (MTUs). Fragments are indicated in different ways
in IPv4 and IPv6:
o In IPv4, fragments are indicated using four fields of the basic
header: Identification (ID), Fragment Offset, a "Don't Fragment"
flag (DF), and a "More Fragments" flag (MF) [RFC791]
o In IPv6, fragments are indicated in an extension header that
includes an ID, Fragment Offset, and M (more fragments) flag
similar to their counterparts in IPv4 [RFC2460]
IPv4 and IPv6 fragmentation differs in a few important ways. IPv6
fragmentation occurs only at the source, so a DF bit is not needed to
prevent downstream devices from initiating fragmentation (i.e., IPv6
always acts as if DF=1). The IPv6 fragment header is present only
when a datagram has been fragmented, or when the source has received
a "packet too big" ICMPv6 error message indicating that the path
cannot support the required minimum 1280-byte IPv6 MTU and is thus
subject to translation [RFC2460][RFC4443]. The latter case is
relevant only for IPv6 datagrams sent to IPv4 destinations to support
subsequent fragmentation after translation to IPv4.
With the exception of these two cases, the ID field is not present
for non-fragmented datagrams, and thus is meaningful only for
datagrams that are already fragmented or datagrams intended to be
fragmented as part of IPv4 translation. Finally, the IPv6 ID field is
32 bits, and required unique per source/destination address pair for
IPv6, whereas for IPv4 it is only 16 bits and required unique per
source/destination/protocol triple.
This document focuses on the IPv4 ID field issues, because in IPv6
the field is larger and present only in fragments.
3.1. Uses of the IPv4 ID Field
The IPv4 ID field was originally intended for fragmentation and
reassembly [RFC791]. Within a given source address, destination
address, and protocol, fragments of an original datagram are matched
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based on their IPv4 ID. This requires that IDs are unique within the
address/protocol triple when fragmentation is possible (e.g., DF=0)
or when it has already occurred (e.g., frag_offset>0 or MF=1).
Other uses have been envisioned for the IPv4 ID field. The field has
been proposed as a way to detect and remove duplicate datagrams,
e.g., at congested routers (noted in Sec. 3.2.1.5 of [RFC1122]) or in
network accelerators. It has similarly been proposed for use at end
hosts to reduce the impact of duplication on higher-layer protocols
(e.g., additional processing in TCP, or the need for application-
layer duplicate suppression in UDP). This is also discussed further
in Section 5.1.
The IPv4 ID field is used in some diagnostic tools to correlate
datagrams measured at various locations along a network path. This is
already insufficient in IPv6 because unfragmented datagrams lack an
ID, so these tools are already being updated to avoid such reliance
on the ID field. This is also discussed further in Section 5.1.
The ID clearly needs to be unique (within MDL, within the
src/dst/protocol tuple) to support fragmentation and reassembly, but
not all datagrams are fragmented or allow fragmentation. This
document deprecates non-fragmentation uses, allowing the ID to be
repeated (within MDL, within the src/dst/protocol tuple) in those
cases.
3.2. Background on IPv4 ID Reassembly Issues
The following is a summary of issues with IPv4 fragment reassembly in
high speed environments raised previously [RFC4963]. Readers are
encouraged to consult RFC 4963 for a more detailed discussion of
these issues.
With the maximum IPv4 datagram size of 64KB, a 16-bit ID field that
does not repeat within 120 seconds means that the aggregate of all
TCP connections of a given protocol between two IP endpoints is
limited to roughly 286 Mbps; at a more typical MTU of 1500 bytes,
this speed drops to 6.4 Mbps [RFC791][RFC1122][RFC4963]. This limit
currently applies for all IPv4 datagrams within a single protocol
(i.e., the IPv4 protocol field) between two IP addresses, regardless
of whether fragmentation is enabled or inhibited, and whether a
datagram is fragmented or not.
IPv6, even at typical MTUs, is capable of 18.7 Tbps with
fragmentation between two IP endpoints as an aggregate across all
protocols, due to the larger 32-bit ID field (and the fact that the
IPv6 next-header field, the equivalent of the IPv4 protocol field, is
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not considered in differentiating fragments). When fragmentation is
not used the field is absent, and in that case IPv6 speeds are not
limited by the ID field uniqueness.
Note also that 120 seconds is only an estimate on the MDL. It is
related to the reassembly timeout as a lower bound and the TCP
Maximum Segment Lifetime as an upper bound (both as noted in
[RFC1122]). Network delays are incurred in other ways, e.g.,
satellite links, which can add seconds of delay even though the TTL
is not decremented by a corresponding amount. There is thus no
enforcement mechanism to ensure that datagrams older than 120 seconds
are discarded.
Wireless Internet devices are frequently connected at speeds over 54
Mbps, and wired links of 1 Gbps have been the default for several
years. Although many end-to-end transport paths are congestion
limited, these devices easily achieve 100+ Mbps application-layer
throughput over LANs (e.g., disk-to-disk file transfer rates), and
numerous throughput demonstrations with COTS systems over wide-area
paths exhibit these speeds for over a decade. This strongly suggests
that IPv4 ID uniqueness has been moot for a long time.
4. Updates to the IPv4 ID Specification
This document updates the specification of the IPv4 ID field in three
distinct ways, as discussed in subsequent subsections:
o Use the IPv4 ID field only for fragmentation
o Avoiding a performance impact when the IPv4 ID field is used
o Encourage safe operation when the IPv4 ID field is used
There are two kinds of datagrams used in the following discussion,
named as follows:
o Atomic datagrams are datagrams not yet fragmented and for which
further fragmentation has been inhibited.
o Non-atomic datagrams are datagrams that either already have been
fragmented or for which fragmentation remains possible.
This same definition can be expressed in pseudo code as using common
logical operators (equals is ==, logical 'and' is &&, logical 'or' is
||, greater than is >, and parenthesis function typically) as:
o Atomic datagrams: (DF==1)&&(MF==0)&&(frag_offset==0)
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o Non-atomic datagrams: (DF==0)||(MF==1)||(frag_offset>0)
The test for non-atomic datagrams is the logical negative of the test
for atomic datagrams, thus all possibilities are considered.
4.1. IPv4 ID Used Only for Fragmentation
Although RFC1122 suggests the IPv4 ID field has other uses, including
datagram de-duplication, such uses are already not interoperable with
known implementations of sources that do not vary their ID. This
document thus defines this field's value only for fragmentation and
reassembly:
>> IPv4 ID field MUST NOT be used for purposes other than
fragmentation and reassembly.
Datagram de-duplication is accomplished using hash-based duplicate
detection for cases where the ID field is absent (IPv6 unfragmented
datagrams), which can also be applied to IPv4 atomic datagrams
without utilizing the ID field [RFC6621].
In atomic datagrams, the IPv4 ID field has no meaning, and thus can
be set to an arbitrary value, i.e., the requirement for non-repeating
IDs within the address/protocol triple is no longer required for
atomic datagrams:
>> Originating sources MAY set the IPv4 ID field of atomic datagrams
to any value.
Second, all network nodes, whether at intermediate routers,
destination hosts, or other devices (e.g., NATs and other address
sharing mechanisms, firewalls, tunnel egresses), cannot rely on the
field:
>> All devices that examine IPv4 headers MUST ignore the IPv4 ID
field of atomic datagrams.
The IPv4 ID field is thus meaningful only for non-atomic datagrams -
datagrams that have either already been fragmented, or those for
which fragmentation remains permitted. Atomic datagrams are detected
by their DF, MF, and fragmentation offset fields as explained in
Section 4, because such a test is completely backward compatible;
this document thus does not reserve any IPv4 ID values, including 0,
as distinguished.
Deprecating the use of the IPv4 ID field for non-reassembly uses
should have little - if any - impact. IPv4 IDs are already frequently
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repeated, e.g., over even moderately fast connections and from some
sources that do not vary the ID at all, and no adverse impact has
been observed. Duplicate suppression was suggested [RFC1122] and has
been implemented in some protocol accelerators, but no impacts of
IPv4 ID reuse have been noted to date. Routers are not required to
issue ICMPs on any particular timescale, and so IPv4 ID repetition
should not have been used for validation and has not been observed,
and again repetition already occurs and would have been noticed
[RFC1812]. ICMP relaying at tunnel ingresses is specified to use soft
state rather than a datagram cache, and should have been noted if the
latter for similar reasons [RFC2003]. These and other legacy issues
are discussed further in Section 5.1.
4.2. Encourage Safe IPv4 ID Use
This document makes further changes to the specification of the IPv4
ID field and its use to encourage its safe use as corollary
requirements changes as follows.
RFC 1122 discusses that if TCP retransmits a segment it may be
possible to reuse the IPv4 ID (see Section 6.2). This can make it
difficult for a source to avoid IPv4 ID repetition for received
fragments. RFC 1122 concludes that this behavior "is not useful";
this document formalizes that conclusion as follows:
>> The IPv4 ID of non-atomic datagrams MUST NOT be reused when
sending a copy of an earlier non-atomic datagram.
RFC 1122 also suggests that fragments can overlap [RFC1122]. Such
overlap can occur if successive retransmissions are fragmented in
different ways but with the same reassembly IPv4 ID. This overlap is
noted as the result of reusing IPv4 IDs when retransmitting
datagrams, which this document deprecates. However, it is also the
result of in-network datagram duplication, which can still occur. As
a result this document does not change the need to support
overlapping fragments.
4.3. IPv4 ID Requirements That Persist
This document does not relax the IPv4 ID field uniqueness
requirements of [RFC791] for non-atomic datagrams, i.e.:
>> Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID
values within one MDL for a given source address/destination
address/protocol triple.
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Such sources include originating hosts, tunnel ingresses, and NATs
(including other address sharing mechanisms) (see Section 5.3).
This document does not relax the requirement that all network devices
honor the DF bit, i.e.:
>> IPv4 datagrams whose DF=1 MUST NOT be fragmented.
>> IPv4 datagram transit devices MUST NOT clear the DF bit.
In specific, DF=1 prevents fragmenting atomic datagrams. DF=1 also
prevents further fragmenting received fragments. In-network
fragmentation is permitted only when DF=0; this document does not
change that requirement.
5. Impact of Proposed Changes
This section discusses the impact of the proposed changes on legacy
devices, datagram generation in updated devices, middleboxes, and
header compression.
5.1. Impact on Legacy Internet Devices
Legacy uses of the IPv4 ID field consist of fragment generation,
fragment reassembly, duplicate datagram detection, and "other" uses.
Current devices already generate ID values that are reused within the
source address, destination address, protocol, and ID tuple in less
than the current estimated Internet MDL of two minutes. They assume
that the MDL over their end-to-end path is much lower.
Existing devices have been known to generate non-varying IDs for
atomic datagrams for nearly a decade, notably some cell phones. Such
constant ID values are the reason for their support as an
optimization of ROHC [RFC5225]. This is discussed further in Section
5.4. Generation of IPv4 datagrams with constant (zero) IDs is also
described as part of the IP/ICMP translation standard [RFC6145].
Many current devices support fragmentation that ignores the IPv4
Don't Fragment (DF) bit. Such devices already transit traffic from
sources that reuse the ID. If fragments of different datagrams
reusing the same ID (within the source/destination/protocol tuple)
arrive at the destination interleaved, fragmentation would fail and
traffic would be dropped. Either such interleaving is uncommon, or
traffic from such devices is not widely traversing these DF-ignoring
devices, because significant occurrence of reassembly errors has not
been reported. DF-ignoring devices do not comply with existing
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standards, and it is not feasible to update the standards to allow
them as compliant.
The ID field has been envisioned for use in duplicate detection, as
discussed in Section 4.1 [RFC1122]. Although this document now allows
IPv4 ID reuse for atomic datagrams, such reuse is already common (as
noted above). Protocol accelerators are known to implement IPv4
duplicate detection, but such devices are also known to violate other
Internet standards to achieve higher end-to-end performance. These
devices would already exhibit erroneous drops for this current
traffic, and this has not been reported.
There are other potential uses of the ID field, such as for
diagnostic purposes. Such uses already need to accommodate atomic
datagrams with reused ID fields. There are no reports of such uses
having problems with current datagrams that reuse IDs. These and any
other uses of the ID field are encouraged to apply IPv6-compatible
methods for IPv4 as well.
Thus, as a result of previous requirements, this document recommends
that IPv4 duplicate detection and diagnostic mechanisms apply IPv6-
compatible methods, i.e., that do not rely on the ID field (e.g., as
suggested in [RFC6621]). This is a consequence of using the ID field
only for reassembly, as well as the known hazard of existing devices
already reusing the ID field.
5.2. Impact on Datagram Generation
The following is a summary of the recommendations that are the result
of the previous changes to the IPv4 ID field specification.
Because atomic datagrams can use arbitrary IPv4 ID values, the ID
field no longer imposes a performance impact in those cases. However,
the performance impact remains for non-atomic datagrams. As a result:
>> Sources of non-atomic IPv4 datagrams MUST rate-limit their output
to comply with the ID uniqueness requirements. Such sources include,
in particular, DNS over UDP [RFC2671].
Because there is no strict definition of the MDL, reassembly hazards
exist regardless of the IPv4 ID reuse interval or the reassembly
timeout. As a result:
>> Higher layer protocols SHOULD verify the integrity of IPv4
datagrams, e.g., using a checksum or hash that can detect reassembly
errors (the UDP checksum is weak in this regard, but better than
nothing).
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Additional integrity checks can be employed using tunnels, as
supported by SEAL, IPsec, or SCTP [RFC4301][RFC4960][RFC5320]. Such
checks can avoid the reassembly hazards that can occur when using UDP
and TCP checksums [RFC4963], or when using partial checksums as in
UDP-Lite [RFC3828]. Because such integrity checks can avoid the
impact of reassembly errors:
>> Sources of non-atomic IPv4 datagrams using strong integrity checks
MAY reuse the ID within MDL values smaller than is typical.
Note, however, that such frequent reuse can still result in corrupted
reassembly and poor throughput, although it would not propagate
reassembly errors to higher layer protocols.
5.3. Impact on Middleboxes
Middleboxes include rewriting devices that include network address
translators (NATs), address/port translators (NAPTs), and other
address sharing mechanisms (ASMs). They also include devices that
inspect and filter datagrams that are not routers, such as
accelerators and firewalls.
The changes proposed in this document may not be implemented by
middleboxes, however these changes are more likely to make current
middlebox behavior compliant than to affect the service provided by
those devices.
5.3.1. Rewriting Middleboxes
NATs and NAPTs rewrite IP fields, and tunnel ingresses (using IPv4
encapsulation) copy and modify some IPv4 fields, so all are
considered sources, as do any devices that rewrite any portion of the
source address, destination address, protocol, and ID tuple for any
datagrams [RFC3022]. This is also true for other ASMs, including 4rd,
IVI, and others in the "A+P" (address plus port) family [Bo11] [De11]
[RFC6219]. It is equally true for any other datagram rewriting
mechanism. As a result, they are subject to all the requirements of
any source, as has been noted.
NATs/ASMs/rewriters present a particularly challenging situation for
fragmentation. Because they overwrite portions of the reassembly
tuple in both directions, they can destroy tuple uniqueness and
result in a reassembly hazard. Whenever IPv4 source address,
destination address, or protocol fields are modified, a
NAT/ASM/rewriter needs to ensure that the ID field is generated
appropriately, rather than simply copied from the incoming datagram.
In specific:
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>> Address sharing or rewriting devices MUST ensure that the IPv4 ID
field of datagrams whose address or protocol are translated comply
with these requirements as if the datagram were sourced by that
device.
This compliance means that the IPv4 ID field of non-atomic datagrams
translated at a NAT/ASM/rewriter needs to obey the uniqueness
requirements of any IPv4 datagram source. Unfortunately, fragments
already violate that requirement, as they repeat an IPv4 ID within
the MDL for a given source address, destination address, and protocol
triple.
Such problems with transmitting fragments through NATs/ASMs/rewriters
are already known; translation is based on the transport port number,
which is present in only the first fragment anyway [RFC3022]. This
document underscores the point that not only is reassembly (and
possibly subsequent fragmentation) required for translation, it can
be used to avoid issues with IPv4 ID uniqueness.
Note that NATs/ASMs already need to exercise special care when
emitting datagrams on their public side, because merging datagrams
from many sources onto a single outgoing source address can result in
IPv4 ID collisions. This situation precedes this document, and is not
affected by it. It is exacerbated in large-scale, so-called "carrier
grade" NATs [Pe11].
Tunnel ingresses act as sources for the outermost header, but tunnels
act as routers for the inner headers (i.e., the datagram as arriving
at the tunnel ingress). Ingresses can always fragment as originating
sources of the outer header, because they control the uniqueness of
that IPv4 ID field and the value of DF on the outer header
independent of those values on the inner (arriving datagram) header.
5.3.2. Filtering Middleboxes
Middleboxes also include devices that filter datagrams, including
network accelerators and firewalls. Some such devices reportedly
feature datagram de-duplication that relies on IP ID uniqueness to
identify duplicates, which has been discussed in Section 5.1.
5.4. Impact on Header Compression
Header compression algorithms already accommodate various ways in
which the IPv4 ID changes between sequential datagrams [RFC1144]
[RFC2508] [RFC3545] [RFC5225]. Such algorithms currently assume that
the IPv4 ID is preserved end-to-end. Some algorithms already allow
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assuming the ID does not change (e.g., ROHC [RFC5225]), where others
include non-changing IDs via zero deltas (e.g., ECRTP [RFC3545]).
When compression assumes a changing ID as a default, having a non-
changing ID can make compression less efficient. Such non-changing
IDs have been described in various RFCs (e.g., footnote 21 of
[RFC1144] and cRTP [RFC2508]). When compression can assume a non-
changing IPv4 ID - as with ROHC and ECRTP - efficiency can be
increased.
5.5. Impact of Network Reordering and Loss
Tolerance to network reordering and loss is a key feature of the
Internet architecture. Although most current IP networks avoid
gratuitous such events, both reordering and loss can and do occur.
Datagrams are already intended to be reordered or lost, and recovery
from those errors (where supported) already occurs at the transport
or higher protocol layers.
Reordering is typically associated with routing transients or where
multiple alternate paths exist. Loss is typically associated with
path congestion or link failure (partial or complete). The impact of
such events is different for atomic and non-atomic datagrams, and is
discussed below. In summary, the recommendations of this document
make the Internet more robust to reordering and loss by emphasizing
the requirements of ID uniqueness for non-atomic datagrams and by
more clearly indicating the impact of these requirements on both
endpoints and datagram transit devices.
5.5.1. Atomic Datagrams Experiencing Reordering or Loss
Reusing ID values does not affect atomic datagrams when the DF bit is
correctly respected, because order restoration does not depend on the
datagram header. TCP uses a transport header sequence number; in some
other protocols, sequence is indicated and restored at the
application layer.
When DF=1 is ignored, reordering or loss can cause fragments of
different datagrams to be interleaved and thus incorrectly
reassembled and thus discarded. Reuse of ID values in atomic packets,
as permitted by this document, can result in higher datagram loss in
such cases. Such cases already can exist because there are known
devices that use a constant ID for atomic packets (some cellphones),
and there are known devices that ignore DF=1, but high levels of
corresponding loss have not been reported. The lack of such reports
indicates either a lack of reordering or loss in such cases, or a
tolerance to the resulting losses. If such issues are reported, it
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would be more productive to address non-compliant devices (that
ignore DF=1), because it is impractical to define Internet
specifications to tolerate devices that ignore those specifications.
This is why this document emphasizes the need to honor DF=1, as well
as that datagram transit devices need to retain the DF bit as
received (i.e., rather than clear it).
5.5.2. Non-atomic Datagrams Experiencing Reordering or Loss
Non-atomic datagrams rely on the uniqueness of the ID value to
tolerate reordering of fragments, notably where fragments of
different datagrams are interleaved as a result of such reordering.
Fragment loss can result in reassembly of fragments from different
origin datagrams, which is why ID reuse in non-atomic datagrams is
based on datagram (fragment) maximum lifetime, not just expected
reordering interleaving.
This document does not change the requirements for uniqueness of IDs
in non-atomic datagrams, and thus does not affect their tolerance to
such reordering or loss. This document emphasizes the need for ID
uniqueness for all datagram sources including rewriting middleboxes,
the need to rate-limit sources to ensure ID uniqueness, the need to
not reuse the ID for retransmitted datagrams, and the need to use
higher-layer integrity checks to prevent reassembly errors - all of
which result in a higher tolerance to reordering or loss events.
6. Updates to Existing Standards
The following sections address the specific changes to existing
protocols indicated by this document.
6.1. Updates to RFC 791
RFC 791 states that:
The originating protocol module of an internet datagram sets the
identification field to a value that must be unique for that
source-destination pair and protocol for the time the datagram
will be active in the internet system.
And later that:
Thus, the sender must choose the Identifier to be unique for this
source, destination pair and protocol for the time the datagram
(or any fragment of it) could be alive in the internet.
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It seems then that a sending protocol module needs to keep a table
of Identifiers, one entry for each destination it has communicated
with in the last maximum datagram lifetime for the internet.
However, since the Identifier field allows 65,536 different
values, some host may be able to simply use unique identifiers
independent of destination.
It is appropriate for some higher level protocols to choose the
identifier. For example, TCP protocol modules may retransmit an
identical TCP segment, and the probability for correct reception
would be enhanced if the retransmission carried the same
identifier as the original transmission since fragments of either
datagram could be used to construct a correct TCP segment.
This document changes RFC 791 as follows:
o IPv4 ID uniqueness applies to only non-atomic datagrams.
o Retransmitted non-atomic IPv4 datagrams are no longer permitted to
reuse the ID value.
6.2. Updates to RFC 1122
RFC 1122 states that:
3.2.1.5 Identification: RFC-791 Section 3.2
When sending an identical copy of an earlier datagram, a
host MAY optionally retain the same Identification field in
the copy.
DISCUSSION:
Some Internet protocol experts have maintained that when a
host sends an identical copy of an earlier datagram, the new
copy should contain the same Identification value as the
original. There are two suggested advantages: (1) if the
datagrams are fragmented and some of the fragments are lost,
the receiver may be able to reconstruct a complete datagram
from fragments of the original and the copies; (2) a
congested gateway might use the IP Identification field (and
Fragment Offset) to discard duplicate datagrams from the
queue.
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This document changes RFC 1122 as follows:
o The IPv4 ID field is no longer permitted to be used for duplicate
detection. This applies to both atomic and non-atomic datagrams.
o Retransmitted non-atomic IPv4 datagrams are no longer permitted to
reuse the ID value.
6.3. Updates to RFC 2003
This document updates how IPv4-in-IPv4 tunnels create IPv4 ID values
for the IPv4 outer header [RFC2003], but only in the same way as for
any other IPv4 datagram source. In specific, RFC 2003 states the
following, where ref. [10] is RFC 791:
Identification, Flags, Fragment Offset
These three fields are set as specified in [10]...
This document changes RFC 2003 as follows:
o The IPv4 ID field is set as permitted by RFCXXXX.
7. Security Considerations
When the IPv4 ID is ignored on receipt (e.g., for atomic datagrams),
its value becomes unconstrained; that field then can more easily be
used as a covert channel. For some atomic datagrams it is now
possible, and may be desirable, to rewrite the IPv4 ID field to avoid
its use as such a channel. Rewriting would be prohibited for
datagrams protected by IPsec Authentication Header (AH), although we
do not recommend use of AH to achieve this result [RFC4302].
The IPv4 ID also now adds much less to the entropy of the header of a
datagram. Such entropy might be used as input to cryptographic
algorithms or pseudorandom generators, although IDs have never been
assured sufficient entropy for such purposes. The IPv4 ID had
previously been unique (for a given source/address pair, and protocol
field) within one MDL, although this requirement was not enforced and
clearly is typically ignored. The IPv4 ID of atomic datagrams is not
required unique, and so contributes no entropy to the header.
The deprecation of the IPv4 ID field's uniqueness for atomic
datagrams can defeat the ability to count devices behind a
NAT/ASM/rewriter [Be02]. This is not intended as a security feature,
however.
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8. IANA Considerations
There are no IANA considerations in this document.
The RFC Editor should remove this section prior to publication
9. References
9.1. Normative References
[RFC791] Postel, J., "Internet Protocol", RFC 791 / STD 5, September
1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", RFC 1122 / STD 3, October 1989.
[RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers",
RFC 1812 / STD 4, Jun. 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119 / BCP 14, March 1997.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
9.2. Informative References
[Be02] Bellovin, S., "A Technique for Counting NATted Hosts",
Internet Measurement Conference, Proceedings of the 2nd ACM
SIGCOMM Workshop on Internet Measurement, Nov. 2002.
[Bo11] Boucadair, M., J. Touch, P. Levis, R. Penno, "Analysis of
Solution Candidates to Reveal a Host Identifier in Shared
Address Deployments", (work in progress), draft-boucadair-
intarea-nat-reveal-analysis, Sept. 2011.
[De11] Despres, R. (Ed.), S. Matsushima, T. Murakami, O. Troan,
"IPv4 Residual Deployment across IPv6-Service networks
(4rd)", (work in progress), draft-despres-intarea-4rd, Mar.
2011.
[Pe11] Perreault, S., (Ed.), I. Yamagata, S. Miyakawa, A.
Nakagawa, H. Ashida, "Common requirements of IP address
sharing schemes", (work in progress), draft-ietf-behave-
lsn-requirements, Mar. 2011.
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[RFC1144] Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, Feb.
1990.
[RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, Dec. 1998.
[RFC2508] Casner, S., V. Jacobson. "Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links", RFC 2508, Feb. 1999.
[RFC2671] Vixie,P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
Aug. 1999.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, Jan. 2001.
[RFC3545] Koren, T., S. Casner, J. Geevarghese, B. Thompson, P.
Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High
Delay, Packet Loss and Reordering", RFC 3545, Jul. 2003.
[RFC3828] Larzon, L-A., M. Degermark, S. Pink, L-E. Jonsson, Ed., G.
Fairhurst, Ed., "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, Jul. 2004.
[RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, Dec. 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, Dec. 2005.
[RFC4443] Conta, A., S. Deering, M. Gupta (Ed.), "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol Version
6 (IPv6) Specification", RFC 4443, March. 2006.
[RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
RFC 4960, Sep. 2007.
[RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly
Errors at High Data Rates," RFC 4963, Jul. 2007.
[RFC5225] Pelletier, G., K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-
Lite", RFC 5225, Apr. 2008.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)", RFC 5320, Feb. 2010.
[RFC6145] Li, X., C. Bao, F. Baker, "IP/ICMP Translation Algorithm,"
RFC 6145, Apr. 2011.
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[RFC6219] Li, X., C. Bao, M. Chen, H. Zhang, J. Wu, "The China
Education and Research Network (CERNET) IVI Translation
Design and Deployment for the IPv4/IPv6 Coexistence and
Transition", RFC 6219, May 2011.
[RFC6621] Macker, J. (Ed.), "Simplified Multicast Forwarding," RFC
6621, May 2012.
10. Acknowledgments
This document was inspired by of numerous discussions among the
authors, Jari Arkko, Lars Eggert, Dino Farinacci, and Fred Templin,
as well as members participating in the Internet Area Working Group.
Detailed feedback was provided by Gorry Fairhurst, Brian Haberman,
Ted Hardie, Mike Heard, Erik Nordmark, Carlos Pignataro, and Dan
Wing. This document originated as an Independent Stream draft co-
authored by Matt Mathis, PSC, and his contributions are greatly
appreciated.
This document was prepared using 2-Word-v2.0.template.dot.
Author's Address
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
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