IP Fragmentation Considered
Fragile
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Internet Area
Internet Area WG
IPv6
Fragmentation
This document describes IP fragmentation and explains how it
introduces fragility to Internet communication.
This document also proposes alternatives to IP fragmentation and
provides recommendations for developers and network operators.
Introduction
Operational experience
reveals that IP fragmentation
introduces fragility to Internet communication. This document describes
IP fragmentation and explains the fragility it introduces. It also
proposes alternatives to IP fragmentation and provides recommendations
for developers and network operators.
While this document identifies issues associated with IP
fragmentation, it does not recommend deprecation. Legacy protocols that
depend upon IP fragmentation would do well to be updated to remove that dependency.
However, some applications and environments (see )
require IP fragmentation. In these cases, the protocol will continue
to rely on IP fragmentation, but the designer should be aware that
fragmented packets may result in black holes. A design should include
appropriate safeguards.
Rather than deprecating IP fragmentation, this document recommends
that upper-layer protocols address the problem of fragmentation at their
layer, reducing their reliance on IP fragmentation to the greatest
degree possible.
Requirements Language
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in BCP 14 when, and only when, they appear in all capitals, as
shown here.
IP Fragmentation
Links, Paths, MTU, and PMTU
An Internet path connects a source node to a destination node. A
path may contain links and routers. If a path contains more than one
link, the links are connected in series, and a router connects each
link to the next.
Internet paths are dynamic. Assume that the path from one node
to another contains a set of links and routers. If a link or a
router fails, the path can also change so that it includes a
different set of links and routers.
Each link is constrained by the number of octets that it can convey in
a single IP packet. This constraint is called the link Maximum
Transmission Unit (MTU). IPv4
requires every link to support an MTU of 68 octets or greater (see NOTE 1).
IPv6 similarly requires every link to
support an MTU of 1280 octets or greater. These are called the IPv4 and IPv6 minimum link MTUs.
Some links, and some ways of using links, result in
additional variable overhead. For the simple case of tunnels,
this document defers to other documents. For other cases,
such as MPLS, this document considers the link MTU to include
appropriate allowance for any such overhead.
Likewise, each Internet path is constrained by the number of octets
that it can convey in a single IP packet. This constraint is called
the Path MTU (PMTU). For any given path, the PMTU is equal to the
smallest of its link MTUs. Because Internet paths are dynamic, PMTU
is also dynamic.
For reasons described below, source nodes estimate the PMTU between
themselves and destination nodes. A source node can produce extremely
conservative PMTU estimates in which:
- The estimate for each IPv4 path is equal to the IPv4 minimum
link MTU.
- The estimate for each IPv6 path is equal to the IPv6 minimum
link MTU.
While these conservative estimates are guaranteed to be less
than or equal to the actual PMTU, they are likely to be much less than
the actual PMTU. This may adversely affect upper-layer protocol
performance.
By executing Path MTU Discovery (PMTUD) procedures
, a source node can
maintain a less conservative estimate of the PMTU between itself and a
destination node. In PMTUD, the source node produces an initial PMTU
estimate. This initial estimate is equal to the MTU of the first link
along the path to the destination node. It can be greater than the
actual PMTU.
Having produced an initial PMTU estimate, the source node sends
non-fragmentable IP packets to the destination node (see NOTE 2). If
one of these packets is larger than the actual PMTU, a downstream
router will not be able to forward the packet through the next link
along the path. Therefore, the downstream router drops the packet and
sends an Internet Control Message Protocol (ICMP)
Packet Too Big (PTB) message to
the source node (see NOTE 3). The ICMP PTB message indicates the MTU
of the link through which the packet could not be forwarded. The
source node uses this information to refine its PMTU estimate.
PMTUD produces a running estimate of the PMTU between a source node
and a destination node. Because PMTU is dynamic, the PMTU estimate can
be larger than the actual PMTU. In order to detect PMTU increases,
PMTUD occasionally resets the PMTU estimate to its initial value and
repeats the procedure described above.
Ideally, PMTUD operates as described above. However, in some
scenarios, PMTUD fails. For example:
- PMTUD relies on the network's ability to deliver ICMP PTB
messages to the source node. If the network cannot deliver ICMP
PTB messages to the source node, PMTUD fails.
- PMTUD is susceptible to attack because ICMP messages are easily
forged and not authenticated by the
receiver. Such attacks can cause PMTUD to produce unnecessarily
conservative PMTU estimates.
- NOTE 1:
- In IPv4, every host must be able to reassemble a packet
whose length is less than or equal to 576 octets. However, the IPv4 minimum
link MTU is not 576. Section
of RFC 791 explicitly states
that the IPv4 minimum link MTU is 68 octets.
- NOTE 2:
- A non-fragmentable packet can be fragmented at its source.
However, it cannot be fragmented by a downstream node. An IPv4 packet
whose Don't Fragment (DF) bit is set to 0 is fragmentable. An IPv4 packet whose
DF bit is set to 1 is non-fragmentable. All IPv6 packets are also
non-fragmentable.
- NOTE 3:
- The ICMP PTB message has two instantiations. In ICMPv4, the ICMP PTB message is a Destination
Unreachable message with Code equal to 4 (fragmentation needed and DF
set). This message was augmented by to
indicate the MTU of the link through which the packet could not be
forwarded. In ICMPv6, the ICMP PTB
message is a Packet Too Big Message with Code equal to 0. This
message also indicates the MTU of the link through which the packet
could not be forwarded.
Fragmentation Procedures
When an upper-layer protocol submits data to the underlying IP
module, and the resulting IP packet's length is greater than the PMTU,
the packet is divided into fragments. Each fragment includes an IP
header and a portion of the original packet.
describes IPv4 fragmentation procedures.
An IPv4 packet whose DF bit is set to 1 may be fragmented by the
source node, but may not be fragmented by a downstream router. An IPv4
packet whose DF bit is set to 0 may be fragmented by the source
node or by a downstream router. When an IPv4 packet is fragmented, all
IP options (which are within the IPv4 header) appear in the first fragment, but only options whose "copy"
bit is set to 1 appear in subsequent fragments.
, notably in
Section , describes
IPv6 fragmentation procedures. An IPv6 packet may be
fragmented only at the source node. When an IPv6 packet is
fragmented, all extension headers appear in the first
fragment, but only per-fragment headers appear in subsequent
fragments. Per-fragment headers include the following:
- The IPv6 header.
- The Hop-by-Hop Options header (if present).
- The Destination Options header (if present and if it precedes a
Routing header).
- The Routing header (if present).
- The Fragment header.
In IPv4, the upper-layer header usually appears in the
first fragment, due to the sizes of the headers involved.
In IPv6, the upper-layer header must appear in the first fragment.
Upper-Layer Reliance on IP Fragmentation
Upper-layer protocols can operate in the following modes:
- Do not rely on IP fragmentation.
- Rely on IP fragmentation by the source node only.
- Rely on IP fragmentation by any node.
Upper-layer protocols running over IPv4 can operate in all of the
above-mentioned modes. Upper-layer protocols running over IPv6 can
operate in the first and second modes only.
Upper-layer protocols that operate in the first two modes (above)
require access to the PMTU estimate. In order to fulfill this
requirement, they can:
- Estimate the PMTU to be equal to the IPv4 or IPv6 minimum link
MTU.
- Access the estimate that PMTUD produced.
- Execute PMTUD procedures themselves.
- Execute Packetization Layer PMTUD (PLPMTUD) procedures
.
According to PLPMTUD procedures, the upper-layer protocol
maintains a running PMTU estimate. It does so by sending probe packets
of various sizes to its upper-layer peer and receiving
acknowledgements. This strategy differs from PMTUD in that it relies
on acknowledgement of received messages, as opposed to ICMP PTB
messages concerning dropped messages. Therefore, PLPMTUD does not rely
on the network's ability to deliver ICMP PTB messages to the
source.
Increased Fragility
This section explains how IP fragmentation introduces fragility to
Internet communication.
Virtual Reassembly
Virtual reassembly is a procedure in which a device
conceptually reassembles a packet, forwards its fragments, and discards
the reassembled copy. In Address plus Port (A+P)
and Carrier Grade NAT (CGN), virtual reassembly
is required in order to correctly translate fragment
addresses. It could be useful to address the problems in Sections
, ,
, and .
Virtual reassembly is computationally expensive and holds
state for indeterminate periods of time. Therefore, it is prone
to errors and attacks.
Policy-Based Routing
IP fragmentation causes problems for routers that implement
policy-based routing.
When a router receives a packet, it identifies the next hop on
route to the packet's destination and forwards the packet to that
next hop. In order to identify the next hop, the router interrogates a
local data structure called the Forwarding Information Base (FIB).
Normally, the FIB contains destination-based entries that map a
destination prefix to a next hop. Policy-based routing allows
destination-based and policy-based entries to coexist in the same FIB.
A policy-based FIB entry maps multiple fields, drawn from either the
IP or transport-layer header, to a next hop.
Policy-Based Routing FIB
Entry |
Type |
Dest. Prefix |
Next Hdr / Dest. Port |
Next Hop |
1 |
Destination-based |
2001:db8::1/128 |
Any / Any |
2001:db8:2::2 |
2 |
Policy-based |
2001:db8::1/128 |
TCP / 80 |
2001:db8:3::3 |
Assume that a router maintains the FIB in . The
first FIB entry is destination-based. It maps a destination prefix
2001:db8::1/128 to a next hop 2001:db8:2::2. The second FIB entry is
policy-based. It maps the same destination prefix 2001:db8::1/128
and a destination port (TCP / 80) to a different next hop
(2001:db8:3::3). The second entry is more specific than the first.
When the router receives the first fragment of a packet that is
destined for TCP port 80 on 2001:db8::1, it interrogates the FIB. Both
FIB entries satisfy the query. The router selects the second FIB entry
because it is more specific and forwards the packet to
2001:db8:3::3.
When the router receives the second fragment of the packet, it
interrogates the FIB again. This time, only the first FIB entry
satisfies the query, because the second fragment contains no
indication that the packet is destined for TCP port 80. Therefore, the
router selects the first FIB entry and forwards the packet to
2001:db8:2::2.
Policy-based routing is also known as filter-based forwarding.
Network Address Translation (NAT)
IP fragmentation causes problems for Network Address Translation
(NAT) devices. When a NAT device detects a new, outbound flow, it maps
that flow's source port and IP address to another source port and IP
address. Having created that mapping, the NAT device translates:
- The source IP address and source port on each outbound
packet.
- The destination IP address and destination port on each inbound
packet.
A+P and
Carrier Grade NAT (CGN)
are two common NAT strategies. In both approaches, the NAT device must virtually
reassemble fragmented packets in order to translate and forward each
fragment.
Stateless Firewalls
As discussed in more detail in , IP
fragmentation causes problems for stateless firewalls whose rules
include TCP and UDP ports. Because port information is only
available in the first fragment and not available
in the subsequent fragments, the firewall is limited to the following
options:
- Accept all subsequent fragments, possibly admitting certain
classes of attack.
- Block all subsequent fragments, possibly blocking legitimate
traffic.
Neither option is attractive.
Equal-Cost Multipath, Link Aggregate Groups, and Stateless Load Balancers
IP fragmentation causes problems for Equal-Cost Multipath (ECMP),
Link Aggregate Groups (LAG), and other stateless load-distribution
technologies. In order to assign a packet or packet fragment to a
link, an intermediate node executes a hash (i.e., load-distributing)
algorithm. The following paragraphs describe a commonly deployed hash
algorithm.
If the packet or packet fragment contains a transport-layer header,
the algorithm accepts the following 5-tuple as input:
- IP Source Address.
- IP Destination Address.
- IPv4 Protocol or IPv6 Next Header.
- transport-layer source port.
- transport-layer destination port.
If the packet or packet fragment does not contain a
transport-layer header, the algorithm accepts only the following
3-tuple as input:
- IP Source Address.
- IP Destination Address.
- IPv4 Protocol or IPv6 Next Header.
Therefore, non-fragmented packets belonging to a flow can be
assigned to one link while fragmented packets belonging to the same
flow can be divided between that link and another. This can cause
suboptimal load distribution.
offers a partial solution to this problem
for IPv6 devices only. According to :
At intermediate routers that perform load distribution, the hash
algorithm used to determine the outgoing component-link in an ECMP
and/or LAG toward the next hop MUST minimally include the 3-tuple
{dest addr, source addr, flow label} and MAY also include the
remaining components of the 5-tuple.
If the algorithm includes only the 3-tuple {dest addr, source addr,
flow label}, it will assign all fragments belonging to a packet to the
same link. (See and ).
In order to avoid the problem described above, implementations
SHOULD implement the recommendations provided in of this document.
IPv4 Reassembly Errors at High Data Rates
IPv4 fragmentation is not sufficiently robust for use under some
conditions in today's Internet. At high data rates, the 16-bit IP
identification field is not large enough to prevent duplicate IDs, resulting in frequent
incorrectly assembled IP fragments, and the TCP and UDP checksums are
insufficient to prevent the resulting corrupted datagrams from being
delivered to upper-layer protocols.
describes some easily reproduced experiments demonstrating the
problem and discusses some of the operational implications of these
observations.
These reassembly issues do not occur as frequently in IPv6 because
the IPv6 identification field is 32 bits long.
Security Vulnerabilities
Security researchers have documented several attacks that exploit
IP fragmentation. The following are examples:
- Overlapping fragment attacks
.
- Resource exhaustion attacks.
- Attacks based on predictable fragment identification values
.
- Evasion of Network Intrusion Detection Systems (NIDS) .
In the overlapping fragment attack, an attacker constructs a series
of packet fragments. The first fragment contains an IP header, a
transport-layer header, and some transport-layer payload. This
fragment complies with local security policy and is allowed to pass
through a stateless firewall. A second fragment, having a nonzero
offset, overlaps with the first fragment. The second fragment also
passes through the stateless firewall. When the packet is reassembled,
the transport-layer header from the first fragment is overwritten by
data from the second fragment. The reassembled packet does not comply
with local security policy. Had it traversed the firewall in one
piece, the firewall would have rejected it.
A stateless firewall cannot protect against the overlapping
fragment attack. However, destination nodes can protect against the
overlapping fragment attack by implementing the procedures described
in RFC 1858, RFC 3128, and RFC 8200. These reassembly procedures detect
the overlap and discard the packet.
The fragment reassembly algorithm is a stateful procedure in an
otherwise stateless protocol. Therefore, it can be exploited by
resource exhaustion attacks. An attacker can construct a series of
fragmented packets with one fragment missing from each packet so that
the reassembly is impossible. Thus, this attack causes resource
exhaustion on the destination node, possibly denying reassembly
services to other flows. This type of attack can be mitigated by
flushing fragment reassembly buffers when necessary, at the expense of
possibly dropping legitimate fragments.
Each IP fragment contains an "Identification" field that
destination nodes use to reassemble fragmented packets. Some
implementations set the Identification field to a predictable value,
thus making it easy for an attacker to forge malicious IP fragments
that would cause the reassembly procedure for legitimate packets to
fail.
NIDS aims at identifying malicious activity by analyzing network
traffic. Ambiguity in the possible result of the fragment reassembly
process may allow an attacker to evade these systems. Many of these
systems try to mitigate some of these evasion techniques (e.g., by
computing all possible outcomes of the fragment reassembly process, at
the expense of increased processing requirements).
PMTU Black-Holing Due to ICMP Loss
As mentioned in , upper-layer protocols can
be configured to rely on PMTUD. Because PMTUD relies upon the network
to deliver ICMP PTB messages, those protocols also rely on the
networks to deliver ICMP PTB messages.
According to , ICMPv6 PTB messages must not
be filtered. However, ICMP PTB delivery is not reliable. It is subject
to both transient and persistent loss.
Transient loss of ICMP PTB messages can cause transient PMTU black
holes. When the conditions contributing to transient loss abate, the
network regains its ability to deliver ICMP PTB messages and
connectivity between the source and destination nodes is restored.
of this document describes conditions that
lead to transient loss of ICMP PTB messages.
Persistent loss of ICMP PTB messages can cause persistent black
holes. Sections , ,
and of this document describe conditions that
lead to persistent loss of ICMP PTB messages.
The problem described in this section is specific to PMTUD. It does
not occur when the upper-layer protocol obtains its PMTU estimate from
PLPMTUD or from any other source.
Transient Loss
The following factors can contribute to transient loss of ICMP
PTB messages:
- Network congestion.
- Packet corruption.
- Transient routing loops.
- ICMP rate limiting.
The effect of rate limiting may be severe, as RFC 4443 recommends
strict rate limiting of ICMPv6 traffic.
Incorrect Implementation of Security Policy
Incorrect implementation of security policy can cause persistent
loss of ICMP PTB messages.
For example, assume that a Customer Premises Equipment (CPE) router implements
the following zone-based security policy:
- Allow any traffic to flow from the inside zone to the outside
zone.
- Do not allow any traffic to flow from the outside zone to the
inside zone unless it is part of an existing flow (i.e., it was
elicited by an outbound packet).
When a correct implementation of the above-mentioned
security policy receives an ICMP PTB message, it examines the ICMP
PTB payload in order to determine whether the original packet (i.e.,
the packet that elicited the ICMP PTB message) belonged to an
existing flow. If the original packet belonged to an existing flow,
the implementation allows the ICMP PTB to flow from the outside zone
to the inside zone. If not, the implementation discards the ICMP PTB
message.
When an incorrect implementation of the above-mentioned security
policy receives an ICMP PTB message, it discards the packet because
its source address is not associated with an existing flow.
The security policy described above has been implemented incorrectly on
many consumer CPE routers.
Persistent Loss Caused by Anycast
Anycast can cause persistent loss of ICMP PTB messages. Consider
the example below:
A DNS client sends a request to an anycast address. The network
routes that DNS request to the nearest instance of that anycast
address (i.e., a DNS server). The DNS server generates a response
and sends it back to the DNS client. While the response does not
exceed the DNS server's PMTU estimate, it does exceed the actual
PMTU.
A downstream router drops the packet and sends an ICMP PTB
message the packet's source (i.e., the anycast address). The network
routes the ICMP PTB message to the anycast instance closest to the
downstream router. That anycast instance may not be the DNS server
that originated the DNS response. It may be another DNS server with
the same anycast address. The DNS server that originated the
response may never receive the ICMP PTB message and may never update
its PMTU estimate.
Persistent Loss Caused by Unidirectional Routing
Unidirectional routing can cause persistent loss of ICMP PTB
messages. Consider the example below:
A source node sends a packet to a destination node. All
intermediate nodes maintain a route to the destination node but do
not maintain a route to the source node. In this case, when an
intermediate node encounters an MTU issue, it cannot send an ICMP
PTB message to the source node.
Black-Holing Due to Filtering or Loss
In RFC 7872, researchers sampled Internet paths to determine
whether they would convey packets that contain IPv6 extension headers.
Sampled paths terminated at popular Internet sites (e.g., popular web,
mail, and DNS servers).
The study revealed that at least 28% of the sampled paths did not
convey packets containing the IPv6 Fragment extension header. In most
cases, fragments were dropped in the destination autonomous system. In
other cases, the fragments were dropped in transit autonomous
systems.
Another study confirmed this
finding. It reported that 37% of sampled endpoints used IPv6-capable
DNS resolvers that were incapable of receiving a fragmented IPv6
response.
It is difficult to determine why network operators drop fragments.
Possible causes follow:
- Hardware inability to process fragmented packets.
- Failure to change vendor defaults.
- Unintentional misconfiguration.
- Intentional configuration (e.g., network operators consciously
chooses to drop IPv6 fragments in order to address the issues
raised in Sections through ,
above.)
Alternatives to IP Fragmentation
Transport-Layer Solutions
The Transport Control Protocol (TCP))
can be operated in a mode that does not require IP fragmentation.
Applications submit a stream of data to TCP. TCP divides that
stream of data into segments, with no segment exceeding the TCP
Maximum Segment Size (MSS). Each segment is encapsulated in a TCP
header and submitted to the underlying IP module. The underlying IP
module prepends an IP header and forwards the resulting packet.
If the TCP MSS is sufficiently small, then the underlying IP module
never produces a packet whose length is greater than the actual PMTU.
Therefore, IP fragmentation is not required.
TCP offers the following mechanisms for MSS management:
- Manual configuration.
- PMTUD.
- PLPMTUD.
Manual configuration is always applicable. If the MSS is configured
to a sufficiently low value, the IP layer will never produce a packet
whose length is greater than the protocol minimum link MTU. However,
manual configuration prevents TCP from taking advantage of larger link
MTUs.
Upper-layer protocols can implement PMTUD in order to discover and
take advantage of larger Path MTUs. However, as mentioned in
, PMTUD relies upon the network to deliver ICMP PTB
messages. Therefore, PMTUD can only provide an estimate of the PMTU in
environments where the risk of ICMP PTB loss is acceptable (e.g.,
known to not be filtered).
By contrast, PLPMTUD does not rely upon the network's ability to
deliver ICMP PTB messages. It utilizes probe messages sent as TCP
segments to determine whether the probed PMTU can be successfully used
across the network path. In PLPMTUD, probing is separated from
congestion control, so that loss of a TCP probe segment does not cause
a reduction of the congestion control window.
defines PLPMTUD procedures for TCP.
While TCP will never knowingly cause the underlying IP module to
emit a packet that is larger than the PMTU estimate, it can cause the
underlying IP module to emit a packet that is larger than the actual
PMTU. For example, if routing changes and as a result the PMTU becomes
smaller, TCP will not know until the ICMP PTB message arrives. If this
occurs, the packet is dropped, the PMTU estimate is updated, the
segment is divided into smaller segments, and each smaller segment is
submitted to the underlying IP module.
The Datagram Congestion Control Protocol
(DCCP) and the Stream Control Transmission
Protocol (SCTP) also can be operated in a mode that does not
require IP fragmentation. They both accept data from an application
and divide that data into segments, with no segment exceeding a
maximum size.
DCCP offers manual configuration,
PMTUD, and PLPMTUD as mechanisms for managing that maximum size.
Datagram protocols can also implement PLPMTUD to estimate the PMTU
via . This proposes
procedures for performing PLPMTUD with UDP, UDP options, SCTP, QUIC,
and other datagram protocols.
Currently, User Datagram Protocol (UDP)
lacks a fragmentation mechanism of its own and relies on IP
fragmentation. However,
proposes a fragmentation mechanism for UDP.
Application-Layer Solutions
recognizes that IP fragmentation reduces
the reliability of Internet communication. It also recognizes that UDP
lacks a fragmentation mechanism of its own and relies on IP
fragmentation. Therefore, offers the
following advice regarding applications the run over the UDP:
An application SHOULD NOT send UDP datagrams that result in IP
packets that exceed the Maximum Transmission Unit (MTU) along the path
to the destination. Consequently, an application SHOULD either use the
path MTU information provided by the IP layer or implement Path MTU
Discovery (PMTUD) itself
to determine whether the path to a
destination will support its desired message size without
fragmentation.
RFC 8085 continues:
Applications that do not follow the recommendation to do
PMTU/PLPMTUD discovery SHOULD still avoid sending UDP datagrams that
would result in IP packets that exceed the path MTU. Because the
actual path MTU is unknown, such applications SHOULD fall back to
sending messages that are shorter than the default effective MTU for
sending (EMTU_S in ). For IPv4, EMTU_S is the
smaller of 576 bytes and the first-hop MTU . For IPv6, EMTU_S is 1280
bytes . The effective PMTU for a directly
connected destination (with no routers on the path) is the configured
interface MTU, which could be less than the maximum link payload size.
Transmission of minimum-sized UDP datagrams is inefficient over paths
that support a larger PMTU, which is a second reason to implement PMTU
discovery.
RFC 8085 assumes that for IPv4 an EMTU_S of 576 is sufficiently
small to be supported by most current Internet
paths, even though the IPv4 minimum link MTU is 68 octets.
This advice applies equally to any application that runs directly
over IP.
Applications That Rely on IPv6 Fragmentation
The following applications rely on IPv6 fragmentation:
- DNS.
- OSPFv2.
- OSPFv3.
- Packet-in-packet encapsulations.
Each of these applications relies on IPv6 fragmentation to a
varying degree. In some cases, that reliance is essential and cannot be
broken without fundamentally changing the protocol. In other cases, that
reliance is incidental, and most implementations already take
appropriate steps to avoid fragmentation.
This list is not comprehensive, and other protocols that rely on IP
fragmentation may exist. They are not specifically considered in the
context of this document.
Domain Name Service (DNS)
DNS relies on UDP for efficiency, and the consequence is the use of
IP fragmentation for large responses, as permitted by the Extension Mechanisms for DNS (EDNS0)
options in the query. It is possible to mitigate the issue of
fragmentation-based packet loss by having queries use smaller EDNS0
UDP buffer sizes or by having the DNS server limit the size of its
UDP responses to some self-imposed maximum packet size that may be
less than the preferred EDNS0 UDP buffer size. In both cases, large
responses are truncated in the DNS, signaling to the client to
re-query using TCP to obtain the complete response. However, the
operational issue of the partial level of support for DNS over TCP,
particularly in the case where IPv6 transport is being used, becomes a
limiting factor of the efficacy of this approach .
Larger DNS responses can normally be avoided by aggressively
pruning the Additional section of DNS responses. One scenario where
such pruning is ineffective is in the use of DNSSEC, where large key
sizes act to increase the response size to certain DNS queries. There
is no effective response to this situation within the DNS other than
using smaller cryptographic keys and adopting of DNSSEC administrative
practices that attempt to keep DNS response as short as possible.
Open Shortest Path First (OSPF)
OSPF implementations can emit messages large enough to cause
fragmentation. However, in order to optimize performance, most OSPF
implementations restrict their maximum message size to a value that
will not cause fragmentation.
Packet-in-Packet Encapsulations
This document acknowledges that in some cases, packets must
be fragmented within IP-in-IP tunnels. Therefore, this document
makes no additional recommendations regarding IP-in-IP
tunnels.
In this document, packet-in-packet encapsulations include
IP-in-IP,
Generic Routing Encapsulation (GRE),
GRE-in-UDP, and
Generic Packet Tunneling in IPv6.
describes
fragmentation issues associated with all of the above-mentioned
encapsulations.
The fragmentation strategy described for GRE in
has been deployed for all of the above-mentioned
encapsulations. This strategy does not rely on IP fragmentation except
in one corner case.
(See
and .)
further
describes this corner case.
See for further
discussion.
UDP Applications Enhancing Performance
Some UDP applications rely on IP fragmentation to achieve
acceptable levels of performance. These applications use UDP datagram
sizes that are larger than the Path MTU so that more data can be
conveyed between the application and the kernel in a single system
call.
To pick one example, the Licklider
Transmission Protocol (LTP), which is in current use on the
International Space Station (ISS), uses UDP datagram sizes larger than
the Path MTU to achieve acceptable levels of performance even though
this invokes IP fragmentation. More generally, SNMP and video
applications may transmit an application-layer quantum of data,
depending on the network layer to fragment and reassemble as
needed.
Recommendations
For Application and Protocol Developers
Developers SHOULD NOT develop new protocols or applications that
rely on IP fragmentation. When a new protocol or application is
deployed in an environment that does not fully support IP
fragmentation, it SHOULD operate correctly, either in its default
configuration or in a specified alternative configuration.
While there may be controlled environments where IP
fragmentation
works reliably, this is a deployment issue and can not be known
to someone developing a new protocol or application. It is not
recommended that new protocols or applications be developed that
rely on IP fragmentation.
Protocols and
applications that rely on IP fragmentation will work less
reliably on the Internet.
Legacy protocols that depend upon IP fragmentation SHOULD be
updated to break that dependency. However, in some cases, there may be
no viable alternative to IP fragmentation (e.g., IPSEC tunnel mode,
IP-in-IP encapsulation).
Applications and protocols cannot necessarily know or control
whether they use lower layers or network paths that rely on such
fragmentation.
In these cases, the protocol will continue to
rely on IP fragmentation but should only be used in environments where
IP fragmentation is known to be supported.
Protocols may be able to avoid IP fragmentation by using a
sufficiently small MTU (e.g., The protocol minimum link MTU), disabling
IP fragmentation, and ensuring that the transport protocol in use
adapts its segment size to the MTU. Other protocols may deploy a
sufficiently reliable PMTU discovery mechanism (e.g., PLPMTUD).
UDP applications SHOULD abide by the recommendations stated in
.
For System Developers
Software libraries SHOULD include provision for PLPMTUD for each
supported transport protocol.
For Middlebox Developers
Middleboxes, which are systems that "transparently"
perform policy functions on passing traffic but do not
participate in the routing system, should process IP fragments
in a manner that is consistent with
and . In many cases, middleboxes
must maintain state in order to achieve this goal.
Price and performance considerations frequently motivate network
operators to deploy stateless middleboxes. These stateless middleboxes
may perform suboptimally, process IP fragments in a manner that is not
compliant with RFC 791 or RFC 8200, or even discard IP fragments
completely. Such behaviors are NOT RECOMMENDED. If a
middlebox implements nonstandard behavior with respect to IP
fragmentation, then that behavior MUST be clearly
documented.
For ECMP, LAG, and Load-Balancer Developers And Operators
In their default configuration, when the IPv6 Flow Label is not
equal to zero, IPv6 devices that implement Equal-Cost Multipath (ECMP)
Routing as described in OSPF
and other routing protocols, Link
Aggregation Grouping (LAG), or other load-distribution
technologies SHOULD accept only the following fields as input to their
hash algorithm:
- IP Source Address.
- IP Destination Address.
- Flow Label.
Operators SHOULD deploy these devices in their
default configuration.
These recommendations are similar to those presented in and . They differ in that
they specify a default configuration.
For Network Operators
Operators MUST ensure proper PMTUD operation in their network,
including making sure the network generates PTB packets when
dropping packets too large compared to outgoing interface
MTU. However, implementations MAY rate limit the generation of
ICMP messages per and .
As per RFC 4890, network operators MUST NOT filter ICMPv6 PTB
messages unless they are known to be forged or otherwise illegitimate.
As stated in , filtering ICMPv6 PTB packets causes
PMTUD to fail. Many upper-layer protocols rely on PMTUD.
As per RFC 8200, network operators MUST NOT deploy IPv6 links whose
MTU is less than 1280 octets.
Network operators SHOULD NOT filter IP fragments if they are known
to have originated at a domain name server or be destined for a domain
name server. This is because domain name services are critical to
operation of the Internet.
IANA Considerations
This document has no IANA actions.
Security Considerations
This document mitigates some of the security considerations
associated with IP fragmentation by discouraging its use. It does not
introduce any new security vulnerabilities, because it does not
introduce any new alternatives to IP fragmentation. Instead, it
recommends well-understood alternatives.
References
Normative References
Packetization Layer Path MTU Discovery for Datagram Transports
Informative References
IPv6, Large UDP Packets and the DNS
Fragmentation Considered Harmful
SIGCOMM '87: Proceedings of the ACM workshop on Frontiers in computer communications technology
Measuring ATR
Insertion, Evasion and Denial of Service: Eluding Network
Intrusion Detection
Secure Networks, Inc.
Secure Networks, Inc.
Acknowledgements
Thanks to ,
, ,
,
,
,
,
, ,
, ,
, ,
, , and
for their comments.