draft-ietf-intarea-ipv4-id-update-05.txt   draft-ietf-intarea-ipv4-id-update-06.txt 
Internet Area WG J. Touch Internet Area WG J. Touch
Internet Draft USC/ISI Internet Draft USC/ISI
Updates: 791,1122,2003 May 30, 2012 Updates: 791,1122,2003 October 9, 2012
Intended status: Proposed Standard Intended status: Proposed Standard
Expires: November 2012 Expires: April 2013
Updated Specification of the IPv4 ID Field Updated Specification of the IPv4 ID Field
draft-ietf-intarea-ipv4-id-update-05.txt draft-ietf-intarea-ipv4-id-update-06.txt
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
This document may contain material from IETF Documents or IETF This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this 10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow material may not have granted the IETF Trust the right to allow
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on November 30, 2012. This Internet-Draft will expire on April 9, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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and RFC2003 to more closely reflect current practice and to more 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 closely match IPv6 so that the field's value is defined only when a
datagram is actually fragmented. It also discusses the impact of datagram is actually fragmented. It also discusses the impact of
these changes on how datagrams are used. these changes on how datagrams are used.
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Conventions used in this document..............................3 2. Conventions used in this document..............................3
3. The IPv4 ID Field..............................................4 3. The IPv4 ID Field..............................................4
4. Uses of the IPv4 ID Field......................................4 3.1. Uses of the IPv4 ID Field.................................4
5. Background on IPv4 ID Reassembly Issues........................5 3.2. Background on IPv4 ID Reassembly Issues...................5
6. Updates to the IPv4 ID Specification...........................6 4. Updates to the IPv4 ID Specification...........................6
6.1. IPv4 ID Used Only for Fragmentation.......................7 4.1. IPv4 ID Used Only for Fragmentation.......................7
6.2. Encourage Safe IPv4 ID Use................................8 4.2. Encourage Safe IPv4 ID Use................................8
6.3. IPv4 ID Requirements That Persist.........................8 4.3. IPv4 ID Requirements That Persist.........................8
7. Impact on Datagram Use.........................................9 5. Impact of Proposed Changes.....................................9
8. Updates to Existing Standards..................................9 5.1. Impact on Legacy Internet Devices.........................9
8.1. Updates to RFC 791.......................................10 5.2. Impact on Datagram Generation............................10
8.2. Updates to RFC 1122......................................10 5.3. Impact on Middleboxes....................................11
8.3. Updates to RFC 2003......................................11 5.3.1. Rewriting Middleboxes...............................11
5.3.2. Filtering Middleboxes...............................12
9. Impact on Middleboxes.........................................11 5.4. Impact on Header Compression.............................13
9.1. Rewriting Middleboxes....................................12 6. Updates to Existing Standards.................................13
9.2. Filtering Middleboxes....................................13 6.1. Updates to RFC 791.......................................13
10. Impact on Header Compression.................................13 6.2. Updates to RFC 1122......................................14
11. Security Considerations......................................13 6.3. Updates to RFC 2003......................................15
12. IANA Considerations..........................................14 7. Security Considerations.......................................15
13. References...................................................14 8. IANA Considerations...........................................15
13.1. Normative References....................................14 9. References....................................................16
13.2. Informative References..................................14 9.1. Normative References.....................................16
14. Acknowledgments..............................................16 9.2. Informative References...................................16
10. Acknowledgments..............................................18
1. Introduction 1. Introduction
In IPv4, the Identification (ID) field is a 16-bit value that is In IPv4, the Identification (ID) field is a 16-bit value that is
unique for every datagram for a given source address, destination unique for every datagram for a given source address, destination
address, and protocol, such that it does not repeat within the address, and protocol, such that it does not repeat within the
Maximum Segment Lifetime (MSL) [RFC791][RFC1122]. As currently maximum datagram lifetime (MDL) [RFC791][RFC1122]. As currently
specified, all datagrams between a source and destination of a given specified, all datagrams between a source and destination of a given
protocol must have unique IPv4 ID values over a period of this MSL, protocol must have unique IPv4 ID values over a period of this MDL,
which is typically interpreted as two minutes (120 seconds). This which is typically interpreted as two minutes, and is related to the
uniqueness is currently specified as for all datagrams, regardless of recommended reassembly timeout [RFC1122]. This uniqueness is
fragmentation settings. currently specified as for all datagrams, regardless of fragmentation
settings.
Uniqueness of the IPv4 ID is commonly violated by high speed devices; Uniqueness of the IPv4 ID is commonly violated by high speed devices;
if strictly enforced, it would limit the speed of a single protocol 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 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 [RFC4963]. It is common for a single connection to operate far in
excess of these rates, which strongly indicates that the uniqueness excess of these rates, which strongly indicates that the uniqueness
of the IPv4 ID as specified is already moot. 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 This document updates the specification of the IPv4 ID field to more
closely reflect current practice, and to include considerations taken closely reflect current practice, and to include considerations taken
into account during the specification of the similar field in IPv6. into account during the specification of the similar field in IPv6.
2. Conventions used in this document 2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119]. document are to be interpreted as described in RFC-2119 [RFC2119].
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o In IPv6, fragments are indicated in an extension header that o In IPv6, fragments are indicated in an extension header that
includes an ID, Fragment Offset, and M (more fragments) flag includes an ID, Fragment Offset, and M (more fragments) flag
similar to their counterparts in IPv4 [RFC2460] similar to their counterparts in IPv4 [RFC2460]
IPv4 and IPv6 fragmentation differs in a few important ways. IPv6 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 fragmentation occurs only at the source, so a DF bit is not needed to
prevent downstream devices from initiating fragmentation (i.e., IPv6 prevent downstream devices from initiating fragmentation (i.e., IPv6
always acts as if DF=1). The IPv6 fragment header is present only 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 when a datagram has been fragmented, or when the source has received
a "packet too big" ICMPv6 error message when the path cannot support a "packet too big" ICMPv6 error message indicating that the path
the required minimum 1280-byte IPv6 MTU and is thus subject to cannot support the required minimum 1280-byte IPv6 MTU and is thus
translation [RFC2460][RFC4443]. The latter case is relevant only for subject to translation [RFC2460][RFC4443]. The latter case is
IPv6 datagrams sent to IPv4 destinations to support subsequent relevant only for IPv6 datagrams sent to IPv4 destinations to support
fragmentation after translation to IPv4. subsequent fragmentation after translation to IPv4.
With the exception of these two cases, the ID field is not present With the exception of these two cases, the ID field is not present
for non-fragmented datagrams, and thus is meaningful only for for non-fragmented datagrams, and thus is meaningful only for
datagrams that are already fragmented or datagrams intended to be datagrams that are already fragmented or datagrams intended to be
fragmented as part of IPv4 translation. Finally, the IPv6 ID field is fragmented as part of IPv4 translation. Finally, the IPv6 ID field is
32 bits, and required unique per source/destination address pair for 32 bits, and required unique per source/destination address pair for
IPv6, whereas for IPv4 it is only 16 bits and required unique per IPv6, whereas for IPv4 it is only 16 bits and required unique per
source/destination/protocol triple. source/destination/protocol triple.
This document focuses on the IPv4 ID field issues, because in IPv6 This document focuses on the IPv4 ID field issues, because in IPv6
the field is larger and present only in fragments. the field is larger and present only in fragments.
4. Uses of the IPv4 ID Field 3.1. Uses of the IPv4 ID Field
The IPv4 ID field was originally intended for fragmentation and The IPv4 ID field was originally intended for fragmentation and
reassembly [RFC791]. Within a given source address, destination reassembly [RFC791]. Within a given source address, destination
address, and protocol, fragments of an original datagram are matched address, and protocol, fragments of an original datagram are matched
based on their IPv4 ID. This requires that IDs are unique within the based on their IPv4 ID. This requires that IDs are unique within the
address/protocol triple when fragmentation is possible (e.g., DF=0) 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). or when it has already occurred (e.g., frag_offset>0 or MF=1).
The IPv4 ID field can be useful for other purposes. The field has Other uses have been envisioned for the IPv4 ID field. The field has
been proposed as a way to detect and remove duplicate datagrams, 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 e.g., at congested routers (noted in Sec. 3.2.1.5 of [RFC1122]) or in
network accelerators. It can similarly be used at end hosts to reduce network accelerators. It has similarly been proposed for use at end
the impact of duplication on higher-layer protocols (e.g., additional hosts to reduce the impact of duplication on higher-layer protocols
processing in TCP, or the need for application-layer duplicate (e.g., additional processing in TCP, or the need for application-
suppression in UDP). layer duplicate suppression in UDP). This is also discussed further
in Section 5.1.
The IPv4 ID field is also used in some debugging tools to correlate The IPv4 ID field is used in some diagnostic tools to correlate
datagrams measured at various locations along a network path. This is datagrams measured at various locations along a network path. This is
already insufficient in IPv6 because unfragmented datagrams lack an already insufficient in IPv6 because unfragmented datagrams lack an
ID, so these tools are already being updated to avoid such reliance ID, so these tools are already being updated to avoid such reliance
on the ID field. on the ID field. This is also discussed further in Section 5.1.
The ID clearly needs to be unique (within MSL, within the The ID clearly needs to be unique (within MDL, within the
src/dst/protocol tuple) to support fragmentation and reassembly, but src/dst/protocol tuple) to support fragmentation and reassembly, but
not all datagrams are fragmented or allow fragmentation. This not all datagrams are fragmented or allow fragmentation. This
document deprecates non-fragmentation uses, allowing the ID to be document deprecates non-fragmentation uses, allowing the ID to be
repeated (within MSL, within the src/dst/protocol tuple) in those repeated (within MDL, within the src/dst/protocol tuple) in those
cases. cases.
5. Background on IPv4 ID Reassembly Issues 3.2. Background on IPv4 ID Reassembly Issues
The following is a summary of issues with IPv4 fragment reassembly in The following is a summary of issues with IPv4 fragment reassembly in
high speed environments raised previously [RFC4963]. Readers are high speed environments raised previously [RFC4963]. Readers are
encouraged to consult RFC 4963 for a more detailed discussion of encouraged to consult RFC 4963 for a more detailed discussion of
these issues. these issues.
With the maximum IPv4 datagram size of 64KB, a 16-bit ID field that 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 does not repeat within 120 seconds means that the aggregate of all
TCP connections of a given protocol between two IP endpoints is TCP connections of a given protocol between two IP endpoints is
limited to roughly 286 Mbps; at a more typical MTU of 1500 bytes, limited to roughly 286 Mbps; at a more typical MTU of 1500 bytes,
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datagram is fragmented or not. datagram is fragmented or not.
IPv6, even at typical MTUs, is capable of 18.7 Tbps with IPv6, even at typical MTUs, is capable of 18.7 Tbps with
fragmentation between two IP endpoints as an aggregate across all fragmentation between two IP endpoints as an aggregate across all
protocols, due to the larger 32-bit ID field (and the fact that the 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 IPv6 next-header field, the equivalent of the IPv4 protocol field, is
not considered in differentiating fragments). When fragmentation is not considered in differentiating fragments). When fragmentation is
not used the field is absent, and in that case IPv6 speeds are not not used the field is absent, and in that case IPv6 speeds are not
limited by the ID field uniqueness. limited by the ID field uniqueness.
Note also that 120 seconds is only an estimate on the maximum Note also that 120 seconds is only an estimate on the MDL. It is
datagram lifetime. It is loosely based on half maximum value of the related to the reassembly timeout as a lower bound and the TCP
IP TTL field (255), measured in seconds, because the TTL was Maximum Segment Lifetime as an upper bound (both as noted in
originally specified as decremented not only for each hop, but also [RFC1122]). Network delays are incurred in other ways, e.g.,
for each second a datagram is held at a router (as implied in satellite links, which can add seconds of delay even though the TTL
[RFC791], although this has long since become a hopcount only). is not decremented by a corresponding amount. There is thus no
Network delays are incurred in other ways, e.g., satellite links, enforcement mechanism to ensure that datagrams older than 120 seconds
which can add seconds of delay even though the TTL is not decremented are discarded.
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 Wireless Internet devices are frequently connected at speeds over 54
Mbps, and wired links of 1 Gbps have been the default for several Mbps, and wired links of 1 Gbps have been the default for several
years. Although many end-to-end transport paths are congestion years. Although many end-to-end transport paths are congestion
limited, these devices easily achieve 100+ Mbps application-layer limited, these devices easily achieve 100+ Mbps application-layer
throughput over LANs (e.g., disk-to-disk file transfer rates), and throughput over LANs (e.g., disk-to-disk file transfer rates), and
numerous throughput demonstrations with COTS systems over wide-area numerous throughput demonstrations with COTS systems over wide-area
paths exhibit these speeds for over a decade. This strongly suggests paths exhibit these speeds for over a decade. This strongly suggests
that IPv4 ID uniqueness has been moot for a long time. that IPv4 ID uniqueness has been moot for a long time.
6. Updates to the IPv4 ID Specification 4. Updates to the IPv4 ID Specification
This document updates the specification of the IPv4 ID field in three This document updates the specification of the IPv4 ID field in three
distinct ways, as discussed in subsequent subsections: distinct ways, as discussed in subsequent subsections:
o Use the IPv4 ID field only for fragmentation o Use the IPv4 ID field only for fragmentation
o Avoiding a performance impact when the IPv4 ID field is used o Avoiding a performance impact when the IPv4 ID field is used
o Encourage safe operation 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, There are two kinds of datagrams used in the following discussion,
named as follows: named as follows:
o Atomic datagrams are datagrams not yet fragmented and for which o Atomic datagrams are datagrams not yet fragmented and for which
further fragmentation has been inhibited. further fragmentation has been inhibited.
o Non-atomic datagrams are datagrams which either have already been o Non-atomic datagrams are datagrams that either already have been
fragmented or for which fragmentation remains possible. fragmented or for which fragmentation remains possible.
This same definition can be expressed in pseudo code as using common This same definition can be expressed in pseudo code as using common
logical operators (equals is ==, logical 'and' is &&, logical 'or' is logical operators (equals is ==, logical 'and' is &&, logical 'or' is
||, greater than is >, and parenthesis function typically) as: ||, greater than is >, and parenthesis function typically) as:
o Atomic datagrams: (DF==1)&&(MF==0)&&(frag_offset==0) o Atomic datagrams: (DF==1)&&(MF==0)&&(frag_offset==0)
o Non-atomic datagrams: (DF==0)||(MF==1)||(frag_offset>0) o Non-atomic datagrams: (DF==0)||(MF==1)||(frag_offset>0)
The test for non-atomic datagrams is the logical negative of the test The test for non-atomic datagrams is the logical negative of the test
for atomic datagrams, thus all possibilities are considered. for atomic datagrams, thus all possibilities are considered.
6.1. IPv4 ID Used Only for Fragmentation 4.1. IPv4 ID Used Only for Fragmentation
Although RFC1122 suggests the IPv4 ID field has other uses, including Although RFC1122 suggests the IPv4 ID field has other uses, including
datagram de-duplication, this document asserts that this field's datagram de-duplication, such uses are already not interoperable with
value is defined only for fragmentation and reassembly: 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 >> IPv4 ID field MUST NOT be used for purposes other than
fragmentation and reassembly. fragmentation and reassembly.
Datagram de-duplication can be accomplished using hash-based Datagram de-duplication is accomplished using hash-based duplicate
duplicate detection for cases where the ID field is absent. 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 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 be set to an arbitrary value, i.e., the requirement for non-repeating
IDs within the address/protocol triple is no longer required for IDs within the address/protocol triple is no longer required for
atomic datagrams: atomic datagrams:
>> Originating sources MAY set the IPv4 ID field of atomic datagrams >> Originating sources MAY set the IPv4 ID field of atomic datagrams
to any value. to any value.
Second, all network nodes, whether at intermediate routers, Second, all network nodes, whether at intermediate routers,
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sharing mechanisms, firewalls, tunnel egresses), cannot rely on the sharing mechanisms, firewalls, tunnel egresses), cannot rely on the
field: field:
>> All devices that examine IPv4 headers MUST ignore the IPv4 ID >> All devices that examine IPv4 headers MUST ignore the IPv4 ID
field of atomic datagrams. field of atomic datagrams.
The IPv4 ID field is thus meaningful only for non-atomic datagrams - The IPv4 ID field is thus meaningful only for non-atomic datagrams -
datagrams that have either already been fragmented, or those for datagrams that have either already been fragmented, or those for
which fragmentation remains permitted. Atomic datagrams are detected which fragmentation remains permitted. Atomic datagrams are detected
by their DF, MF, and fragmentation offset fields as explained in by their DF, MF, and fragmentation offset fields as explained in
Section 6, because such a test is completely backward compatible; Section 4, because such a test is completely backward compatible;
this document thus does not reserve any IPv4 ID values, including 0, this document thus does not reserve any IPv4 ID values, including 0,
as distinguished. as distinguished.
Deprecating the use of the IPv4 ID field for non-reassembly uses Deprecating the use of the IPv4 ID field for non-reassembly uses
should have little - if any - impact. IPv4 IDs are already frequently should have little - if any - impact. IPv4 IDs are already frequently
repeated, e.g., over even moderately fast connections. Duplicate repeated, e.g., over even moderately fast connections and from some
suppression was only suggested [RFC1122], and no impacts of IPv4 ID sources that do not vary the ID at all, and no adverse impact has
reuse have been noted. Routers are not required to issue ICMPs on any been observed. Duplicate suppression was suggested [RFC1122] and has
particular timescale, and so IPv4 ID repetition should not have been been implemented in some protocol accelerators, but no impacts of
used for validation, and again repetition occurs and probably could IPv4 ID reuse have been noted to date. Routers are not required to
have been noticed [RFC1812]. ICMP relaying at tunnel ingresses is issue ICMPs on any particular timescale, and so IPv4 ID repetition
specified to use soft state rather than a datagram cache, and should should not have been used for validation and has not been observed,
have been noted if the latter for similar reasons [RFC2003]. 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.
6.2. Encourage Safe IPv4 ID Use 4.2. Encourage Safe IPv4 ID Use
This document makes further changes to the specification of the IPv4 This document makes further changes to the specification of the IPv4
ID field and its use to encourage its safe use as corollary ID field and its use to encourage its safe use as corollary
requirements changes as follows. requirements changes as follows.
RFC 1122 discusses that if TCP retransmits a segment it may be RFC 1122 discusses that if TCP retransmits a segment it may be
possible to reuse the IPv4 ID (see Section 8.2). This can make it 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 difficult for a source to avoid IPv4 ID repetition for received
fragments. RFC 1122 concludes that this behavior "is not useful"; fragments. RFC 1122 concludes that this behavior "is not useful";
this document formalizes that conclusion as follows: this document formalizes that conclusion as follows:
>> The IPv4 ID of non-atomic datagrams MUST NOT be reused when >> The IPv4 ID of non-atomic datagrams MUST NOT be reused when
sending a copy of an earlier non-atomic datagram. sending a copy of an earlier non-atomic datagram.
RFC 1122 also suggests that fragments can overlap [RFC1122]. Such RFC 1122 also suggests that fragments can overlap [RFC1122]. Such
overlap can occur if successive retransmissions are fragmented in overlap can occur if successive retransmissions are fragmented in
different ways but with the same reassembly IPv4 ID. This overlap is different ways but with the same reassembly IPv4 ID. This overlap is
noted as the result of reusing IPv4 IDs when retransmitting noted as the result of reusing IPv4 IDs when retransmitting
datagrams, which this document deprecates. However, it is also the datagrams, which this document deprecates. However, it is also the
result of in-network datagram duplication, which can still occur. As result of in-network datagram duplication, which can still occur. As
a result this document does not change the need to support a result this document does not change the need to support
overlapping fragments. overlapping fragments.
6.3. IPv4 ID Requirements That Persist 4.3. IPv4 ID Requirements That Persist
This document does not relax the IPv4 ID field uniqueness This document does not relax the IPv4 ID field uniqueness
requirements of [RFC791] for non-atomic datagrams, i.e.: requirements of [RFC791] for non-atomic datagrams, i.e.:
>> Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID >> Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID
values within one MSL for a given source address/destination values within one MDL for a given source address/destination
address/protocol triple. address/protocol triple.
Such sources include originating hosts, tunnel ingresses, and NATs Such sources include originating hosts, tunnel ingresses, and NATs
(including other address sharing mechanisms) (see Section 9). (including other address sharing mechanisms) (see Section 5.3).
This document does not relax the requirement that all network devices This document does not relax the requirement that all network devices
honor the DF bit, i.e.: honor the DF bit, i.e.:
>> IPv4 datagrams whose DF=1 MUST NOT be fragmented. >> IPv4 datagrams whose DF=1 MUST NOT be fragmented.
>> IPv4 datagram transit devices MUST NOT clear the DF bit. >> IPv4 datagram transit devices MUST NOT clear the DF bit.
In specific, DF=1 prevents fragmenting atomic datagrams. DF=1 also In specific, DF=1 prevents fragmenting atomic datagrams. DF=1 also
prevents further fragmenting received fragments. In-network prevents further fragmenting received fragments. In-network
fragmentation is permitted only when DF=0; this document does not fragmentation is permitted only when DF=0; this document does not
change that requirement. change that requirement.
7. Impact on Datagram Use 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
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 The following is a summary of the recommendations that are the result
of the previous changes to the IPv4 ID field specification. of the previous changes to the IPv4 ID field specification.
Because atomic datagrams can use arbitrary IPv4 ID values, the ID Because atomic datagrams can use arbitrary IPv4 ID values, the ID
field no longer imposes a performance impact in those cases. However, field no longer imposes a performance impact in those cases. However,
the performance impact remains for non-atomic datagrams. As a result: the performance impact remains for non-atomic datagrams. As a result:
>> Sources of non-atomic IPv4 datagrams MUST rate-limit their output >> Sources of non-atomic IPv4 datagrams MUST rate-limit their output
to comply with the ID uniqueness requirements. to comply with the ID uniqueness requirements.
Such sources include, in particular, DNS over UDP [RFC2671]. Such sources include, in particular, DNS over UDP [RFC2671].
Because there is no strict definition of the MSL, reassembly hazards Because there is no strict definition of the MDL, reassembly hazards
exist regardless of the IPv4 ID reuse interval or the reassembly exist regardless of the IPv4 ID reuse interval or the reassembly
timeout. As a result: timeout. As a result:
>> Higher layer protocols SHOULD verify the integrity of IPv4 >> Higher layer protocols SHOULD verify the integrity of IPv4
datagrams, e.g., using a checksum or hash that can detect reassembly datagrams, e.g., using a checksum or hash that can detect reassembly
errors (the UDP checksum is weak in this regard, but better than errors (the UDP checksum is weak in this regard, but better than
nothing), as in SEAL [RFC5320]. nothing).
Additional integrity checks can be employed using tunnels, as in Additional integrity checks can be employed using tunnels, as
SEAL, IPsec, or SCTP [RFC4301][RFC4960][RFC5320]. Such checks can supported by SEAL, IPsec, or SCTP [RFC4301][RFC4960][RFC5320]. Such
avoid the reassembly hazards that can occur when using UDP and TCP checks can avoid the reassembly hazards that can occur when using UDP
checksums [RFC4963], or when using partial checksums as in UDP-Lite and TCP checksums [RFC4963], or when using partial checksums as in
[RFC3828]. Because such integrity checks can avoid the impact of UDP-Lite [RFC3828]. Because such integrity checks can avoid the
reassembly errors: impact of reassembly errors:
>> Sources of non-atomic IPv4 datagrams using strong integrity checks >> Sources of non-atomic IPv4 datagrams using strong integrity checks
MAY reuse the ID within MSL values smaller than is typical. MAY reuse the ID within MDL values smaller than is typical.
Note, however, that such frequent reuse can still result in corrupted Note, however, that such frequent reuse can still result in corrupted
reassembly and poor throughput, although it would not propagate reassembly and poor throughput, although it would not propagate
reassembly errors to higher layer protocols. reassembly errors to higher layer protocols.
8. Updates to Existing Standards 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:
>> 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
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.
6. Updates to Existing Standards
The following sections address the specific changes to existing The following sections address the specific changes to existing
protocols indicated by this document. protocols indicated by this document.
8.1. Updates to RFC 791 6.1. Updates to RFC 791
RFC 791 states that: RFC 791 states that:
The originating protocol module of an internet datagram sets the The originating protocol module of an internet datagram sets the
identification field to a value that must be unique for that identification field to a value that must be unique for that
source-destination pair and protocol for the time the datagram source-destination pair and protocol for the time the datagram
will be active in the internet system. will be active in the internet system.
And later that: And later that:
skipping to change at page 10, line 42 skipping to change at page 14, line 19
identifier as the original transmission since fragments of either identifier as the original transmission since fragments of either
datagram could be used to construct a correct TCP segment. datagram could be used to construct a correct TCP segment.
This document changes RFC 791 as follows: This document changes RFC 791 as follows:
o IPv4 ID uniqueness applies to only non-atomic datagrams. o IPv4 ID uniqueness applies to only non-atomic datagrams.
o Retransmitted non-atomic IPv4 datagrams are no longer permitted to o Retransmitted non-atomic IPv4 datagrams are no longer permitted to
reuse the ID value. reuse the ID value.
8.2. Updates to RFC 1122 6.2. Updates to RFC 1122
RFC 1122 states that: RFC 1122 states that:
3.2.1.5 Identification: RFC-791 Section 3.2 3.2.1.5 Identification: RFC-791 Section 3.2
When sending an identical copy of an earlier datagram, a When sending an identical copy of an earlier datagram, a
host MAY optionally retain the same Identification field in host MAY optionally retain the same Identification field in
the copy. the copy.
DISCUSSION: DISCUSSION:
skipping to change at page 11, line 26 skipping to change at page 15, line 5
queue. queue.
This document changes RFC 1122 as follows: This document changes RFC 1122 as follows:
o The IPv4 ID field is no longer permitted to be used for duplicate o The IPv4 ID field is no longer permitted to be used for duplicate
detection. This applies to both atomic and non-atomic datagrams. detection. This applies to both atomic and non-atomic datagrams.
o Retransmitted non-atomic IPv4 datagrams are no longer permitted to o Retransmitted non-atomic IPv4 datagrams are no longer permitted to
reuse the ID value. reuse the ID value.
8.3. Updates to RFC 2003 6.3. Updates to RFC 2003
This document updates how IPv4-in-IPv4 tunnels create IPv4 ID values 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 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 any other IPv4 datagram source. In specific, RFC 2003 states the
following, where ref. [10] is RFC 791: following, where ref. [10] is RFC 791:
Identification, Flags, Fragment Offset Identification, Flags, Fragment Offset
These three fields are set as specified in [10]... These three fields are set as specified in [10]...
This document changes RFC 2003 as follows: This document changes RFC 2003 as follows:
o The IPv4 ID field is set as permitted by this document. o The IPv4 ID field is set as permitted by RFCXXXX.
9. 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.
9.1. 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:
>> 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 MSL 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.
9.2. Filtering Middleboxes
Middleboxes also include devices that filter datagrams, including
network accelerators and firewalls. Some such devices reportedly
feature datagram de-duplication, which relies on IP ID uniqueness to
identify duplicates. Such accelerators already risk dropping non-
duplicate datagrams because of early ID reuse, and, as a result of
earlier requirements:
>> Datagram de-duplication mechanisms MUST ignore the ID values on
atomic datagrams.
10. 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
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.
11. Security Considerations 7. Security Considerations
When the IPv4 ID is ignored on receipt (e.g., for atomic datagrams), When the IPv4 ID is ignored on receipt (e.g., for atomic datagrams),
its value becomes unconstrained; that field then can more easily be its value becomes unconstrained; that field then can more easily be
used as a covert channel. For some atomic datagrams - notably those used as a covert channel. For some atomic datagrams it is now
not protected by IPsec Authentication Header (AH) [RFC4302] - it is possible, and may be desirable, to rewrite the IPv4 ID field to avoid
now possible, and may be desirable, to rewrite the IPv4 ID field to its use as such a channel. Rewriting would be prohibited for
avoid its use as such a channel. 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 entropy of the header of a The IPv4 ID also now adds much less to the entropy of the header of a
datagram. The IPv4 ID had previously been unique (for a given datagram. Such entropy might be used as input to cryptographic
source/address pair, and protocol field) within one MSL, although algorithms or pseudorandom generators, although IDs have never been
this requirement was not enforced and clearly is typically ignored. assured sufficient entropy for such purposes. The IPv4 ID had
The IPv4 ID of atomic datagrams is not required unique, and so previously been unique (for a given source/address pair, and protocol
contributes no entropy to the header. 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 The deprecation of the IPv4 ID field's uniqueness for atomic
datagrams can defeat the ability to count devices behind a datagrams can defeat the ability to count devices behind a
NAT/ASM/rewriter [Be02]. This is not intended as a security feature, NAT/ASM/rewriter [Be02]. This is not intended as a security feature,
however. however.
12. IANA Considerations 8. IANA Considerations
There are no IANA considerations in this document. There are no IANA considerations in this document.
The RFC Editor should remove this section prior to publication The RFC Editor should remove this section prior to publication
13. References 9. References
13.1. Normative References 9.1. Normative References
[RFC791] Postel, J., "Internet Protocol", RFC 791 / STD 5, September [RFC791] Postel, J., "Internet Protocol", RFC 791 / STD 5, September
1981. 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", RFC 1122 / STD 3, October 1989. Communication Layers", RFC 1122 / STD 3, October 1989.
[RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers", [RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers",
RFC 1812 / STD 4, Jun. 1995. RFC 1812 / STD 4, Jun. 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119 / BCP 14, March 1997. Requirement Levels", RFC 2119 / BCP 14, March 1997.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996. October 1996.
13.2. Informative References 9.2. Informative References
[Be02] Bellovin, S., "A Technique for Counting NATted Hosts", [Be02] Bellovin, S., "A Technique for Counting NATted Hosts",
Internet Measurement Conference, Proceedings of the 2nd ACM Internet Measurement Conference, Proceedings of the 2nd ACM
SIGCOMM Workshop on Internet Measurement, Nov. 2002. SIGCOMM Workshop on Internet Measurement, Nov. 2002.
[Bo11] Boucadair, M., J. Touch, P. Levis, R. Penno, "Analysis of [Bo11] Boucadair, M., J. Touch, P. Levis, R. Penno, "Analysis of
Solution Candidates to Reveal a Host Identifier in Shared Solution Candidates to Reveal a Host Identifier in Shared
Address Deployments", (work in progress), draft-boucadair- Address Deployments", (work in progress), draft-boucadair-
intarea-nat-reveal-analysis, Sept. 2011. intarea-nat-reveal-analysis, Sept. 2011.
skipping to change at page 16, line 12 skipping to change at page 17, line 44
[RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly [RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly
Errors at High Data Rates," RFC 4963, Jul. 2007. Errors at High Data Rates," RFC 4963, Jul. 2007.
[RFC5225] Pelletier, G., K. Sandlund, "RObust Header Compression [RFC5225] Pelletier, G., K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP- Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-
Lite", RFC 5225, Apr. 2008. Lite", RFC 5225, Apr. 2008.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)", RFC 5320, Feb. 2010. Adaptation Layer (SEAL)", RFC 5320, Feb. 2010.
[RFC6145] Li, X., C. Bao, F. Baker, "IP/ICMP Translation Algorithm,"
RFC 6145, Apr. 2011.
[RFC6219] Li, X., C. Bao, M. Chen, H. Zhang, J. Wu, "The China [RFC6219] Li, X., C. Bao, M. Chen, H. Zhang, J. Wu, "The China
Education and Research Network (CERNET) IVI Translation Education and Research Network (CERNET) IVI Translation
Design and Deployment for the IPv4/IPv6 Coexistence and Design and Deployment for the IPv4/IPv6 Coexistence and
Transition", RFC 6219, May 2011. Transition", RFC 6219, May 2011.
14. Acknowledgments [RFC6621] Macker, J. (Ed.), "Simplified Multicast Forwarding," RFC
6621, May 2012.
10. Acknowledgments
This document was inspired by of numerous discussions among the This document was inspired by of numerous discussions among the
authors, Jari Arkko, Lars Eggert, Dino Farinacci, and Fred Templin, authors, Jari Arkko, Lars Eggert, Dino Farinacci, and Fred Templin,
as well as members participating in the Internet Area Working Group. as well as members participating in the Internet Area Working Group.
Detailed feedback was provided by Gorry Fairhurst, Brian Haberman, Detailed feedback was provided by Gorry Fairhurst, Brian Haberman,
Ted Hardie, Mike Heard, Erik Nordmark, Carlos Pignataro, and Dan Ted Hardie, Mike Heard, Erik Nordmark, Carlos Pignataro, and Dan
Wing. This document originated as an Independent Stream draft co- Wing. This document originated as an Independent Stream draft co-
authored by Matt Mathis, PSC, and his contributions are greatly authored by Matt Mathis, PSC, and his contributions are greatly
appreciated. appreciated.
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