Internet DRAFT - draft-bonica-intarea-frag-fragile
draft-bonica-intarea-frag-fragile
Internet Area WG R. Bonica
Internet-Draft Juniper Networks
Intended status: Best Current Practice F. Baker
Expires: January 24, 2019 Unaffiliated
G. Huston
APNIC
R. Hinden
Check Point Software
O. Troan
Cisco
F. Gont
SI6 Networks
July 23, 2018
IP Fragmentation Considered Fragile
draft-bonica-intarea-frag-fragile-03
Abstract
This document provides an overview of IP fragmentation. It also
explains how IP fragmentation reduces the reliability of Internet
communication.
Finally, this document proposes alternatives to IP fragmentation and
provides recommendations for application developers and network
operators.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 24, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. IP Fragmentation . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Links, Paths, MTU and PMTU . . . . . . . . . . . . . . . 3
2.2. Upper-layer Protocols . . . . . . . . . . . . . . . . . . 5
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 7
4. IP Fragmentation Reduces Reliability . . . . . . . . . . . . 7
4.1. Middle Box Failures . . . . . . . . . . . . . . . . . . . 7
4.2. Partial Filtering . . . . . . . . . . . . . . . . . . . . 8
4.3. Telemetry and Monitoring and monitoring Failures . . . . 8
4.4. Suboptimal Load Balancing . . . . . . . . . . . . . . . . 9
4.5. Security Vulnerabilities . . . . . . . . . . . . . . . . 9
4.6. Blackholing Due to ICMP Loss . . . . . . . . . . . . . . 11
4.6.1. Transient Loss . . . . . . . . . . . . . . . . . . . 12
4.6.2. Incorrect Implementation of Security Policy . . . . . 12
4.6.3. Persistant Loss Caused By Anycast . . . . . . . . . . 13
4.7. Blackholing Due To Filtering . . . . . . . . . . . . . . 13
5. Alternatives to IP Fragmentation . . . . . . . . . . . . . . 14
5.1. Transport Layer Solutions . . . . . . . . . . . . . . . . 14
5.2. Application Layer Solutions . . . . . . . . . . . . . . . 15
6. Applications That Rely on IPv6 Fragmentation . . . . . . . . 16
6.1. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3. Packet-in-Packet Encapsulations . . . . . . . . . . . . . 17
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. For Application Developers . . . . . . . . . . . . . . . 17
7.2. For Network Operators . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
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11.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Contributors' Address . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Operational experience [RFC7872] [Huston] reveals that IP
fragmentation reduces the reliability of Internet communication.
This document provides an overview of IP fragmentation. It also
explains how IP fragmentation reduces the reliability of Internet
communication.
Finally, this document proposes alternatives to IP fragmentation and
provides recommendations for application developers and network
operators.
2. IP Fragmentation
2.1. Links, Paths, MTU and PMTU
An Internet path connects a source node to a destination node. A
path can contain links and intermediate systems. If a path contains
more than one link, the links are connected in series and an
intermediate system connects each link to the next. An intermediate
system can be a router or a middle box.
Internet paths are dynamic. Assume that the path from one node to
another contains a set of links and intermediate systems. If the
network topology changes, that path can also change so that it
includes a different set of links and intermediate systems.
Each link is constrained by the number of bytes that it can convey in
a single IP packet. This constraint is called the link Maximum
Transmission Unit (MTU). IPv4 [RFC0791] requires every link to have
an MTU of 68 bytes or greater. IPv6 [RFC8200] requires every link to
have an MTU of 1280 bytes or greater. These are called the IPv4 and
IPv6 minimum link MTU's.
Each Internet path is constrained by the number of bytes that it can
convey in a IP single packet. This constraint is called the Path MTU
(PMTU). For any given path, the PMTU is equal to the smallest of its
link MTU's. 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:
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o The estimate for each IPv4 path is equal to the IPv4 minimum link
MTU.
o 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) [RFC1191] [RFC8201]
procedures, a source node can maintain a less conservative, running
estimate of the PMTU between itself and a destination node.
According to these procedures, 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. 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) [RFC0792] [RFC4443] Packet
Too Big (PTB) message to the source node. 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, at any given time,
the PMTU estimate can differ from the actual PMTU. In order to
detect PMTU increases, PMTUD occasionally resets the PMTU estimate to
the MTU of the first link along path to the destination node. It
then repeats the procedure described above.
PMTUD has the following characteristics:
o It relies on the network's ability to deliver ICMP PTB messages to
the source node.
o It is susceptible to attack because ICMP messages are easily
forged [RFC5927].
FOOTNOTE: According to RFC 0791, every IPv4 host must be capable of
receiving a packet whose length is equal to 576 bytes. However, the
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IPv4 minimum link MTU is not 576. Section 3.2 of RFC 0791 explicitly
states that the IPv4 minimum link MTU is 68 bytes.
FOOTNOTE: In the paragraphs above, the term "non-fragmentable packet"
is introduced. A non-fragmentable packet can be fragmented at its
source. However, it cannot be fragmented by a downstream node. An
IPv4 packet whose DF-bit is set to zero is fragmentable. An IPv4
packet whose DF-bit is set to one is non-fragmentable. All IPv6
packets are also non-fragmentable.
FOOTNOTE: In the paragraphs above, the term "ICMP PTB message" is
introduced. The ICMP PTB message has two instantiations. In ICMPv4
[RFC0792], the ICMP PTB message is Destination Unreachable message
with Code equal to (4) fragmentation needed and DF set. This message
was augmented by [RFC1191] to indicates the MTU of the link through
which the packet could not be forwarded. In ICMPv6 [RFC4443], 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.
2.2. Upper-layer Protocols
When an upper-layer protocol submits data to the underlying IP
module, and the resulting IP packet's length is greater than the
PMTU, IP fragmentation may be required. IP fragmentation divides a
packet into fragments. Each fragment includes an IP header and a
portion of the original packet.
[RFC0791] describes IPv4 fragmentation procedures. IPv4 packets
whose DF-bit is set to one cannot be fragmented. IPv4 packets whose
DF-bit is set to zero can be fragmented at the source node or by any
downstream router. [RFC8200] describes IPv6 fragmentation
procedures. IPv6 packets can be fragmented at the source node only.
IPv4 fragmentation differs slightly from IPv6 fragmentation.
However, in both IP versions, the upper-layer header appears in the
first fragment only. It does not appear in subsequent fragments.
Upper-layer protocols can operate in the following modes:
o Do not rely on IP fragmentation.
o Rely on IP source fragmentation only (i.e., fragmentation at the
source node).
o Rely on IP source fragmentation and downstream fragmentation
(i.e., fragmentation at any node along the path).
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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 fulfil this
requirement, they can
o Estimate the PMTU to be equal to the IPv4 or IPv6 minimum link
MTU.
o Access the estimate that PMTUD produced.
o Execute PMTUD procedures themselves.
o Execute Packetization Layer PMTUD (PLPMTUD) [RFC4821]
[I-D.fairhurst-tsvwg-datagram-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 peer and receiving acknowledgements. This
strategy differs from PMTUD in that it relies of 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.
An upper-layer protocol that does not rely on IP fragmentation never
causes the underlying IP module to emit
o A fragmentable IP packet (i.e., an IPv4 packet with the DF-bit set
to zero).
o An IP fragment.
o A packet whose length is greater than the PMTU estimate.
However, when the PMTU estimate is greater than the actual PMTU, the
upper-layer protocol can cause the underlying IP module to emit a
packet whose length is greater than the actual PMTU. When this
occurs, a downstream router drops the packet and the source node
refines its PMTU estimate, employing either PMTUD or PLPMTUD
procedures.
When an upper-layer protocol that relies on IP source fragmentation
only submits data to the underlying IP module, and the resulting
packet is larger than the PMTU estimate, the underlying IP module
fragments the packet and emits the fragments. However, the upper-
layer protocol never causes the underlying IP module to emit
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o A fragmentable IP packet.
o A packet whose length is greater than the PMTU estimate.
When the PMTU estimate is greater than the actual PMTU, the upper-
layer protocol can cause the underlying IP module to emit a packet
whose length is greater than the actual PMTU. When this occurs, a
downstream router drops the packet and the source node refines its
PMTU estimate, employing either PMTUD or PLPMTUD procedures.
An upper-layer protocol that relies on IP source fragmentation and
downstream fragmentation can cause the underlying IP module to emit
o A fragmentable IP packet.
o An IP fragment.
o A packet whose length is greater than the PMTU estimate.
A protocol that relies on IP source fragmentation and downstream
fragmentation does not require access to the PMTU estimate. For
these protocols, the underlying IP module:
o Fragments all packets whose length exceeds the MTU of the first
link along the path to the destination.
o Sets the DF-bit to zero, so that downstream nodes can fragment the
packet.
3. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
4. IP Fragmentation Reduces Reliability
This section explains how IP fragmentation reduces the reliability of
Internet communication.
4.1. Middle Box Failures
Many middle boxes require access to the transport-layer header.
However, when a packet is divided into fragments, the transport-layer
header appears in the first fragment only. It does not appear in
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subsequent fragments. This omission can prevent middle boxes from
delivering their intended services.
For example, assume that a router diverts selected packets from their
normal path towards network appliances that support deep packet
inspection and lawful intercept. The router selects packets for
diversion based upon the following 5-tuple:
o IP Source Address.
o IP Destination Address.
o IPv4 Protocol or IPv6 Next Header.
o transport-layer source port.
o transport-layer destination port.
IP fragmentation causes this selection algorithm to behave
suboptimally, because the transport-layer header appears only in the
first fragment of each packet.
In another example, a middle box remarks a packet's Differentiated
Services Code Point [RFC2474] based upon the above-mentioned 5-tuple.
IP fragmentation causes this process to behave suboptimally, because
the transport-layer header appears only in the first fragment of each
packet.
In all of the above-mentioned examples, the middle box cannot deliver
its intended service without reassembling fragmented packets.
4.2. Partial Filtering
IP fragments cause problems for firewalls whose filter rules include
decision making based on TCP and UDP ports. As the port information
is not in the trailing fragments the firewall may elect to accept all
trailing fragments, which may admit certain classes of attack, or may
elect to block all trailing fragments, which may block otherwise
legitimate traffic, or may elect to reassemble all fragmented
packets, which may be inefficient and negatively affect performance.
4.3. Telemetry and Monitoring and monitoring Failures
Stateless telemetry and monitoring strategies may require the
transport-layer header to appear in every packet. However, when a
packet is divided into fragments, the transport-layer header appears
in the first fragment only. It does not appear in subsequent
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fragments. This omission can prevent some stateless telemetry
strategies from functioning correctly.
4.4. Suboptimal Load Balancing
Many stateless load-balancers require access to the transport-layer
header. Assume that a load-balancer distributes flows among parallel
links. In order to optimize load balancing, the load-balancer sends
every packet or packet fragment belonging to a flow through the same
link.
In order to assign a packet or packet fragment to a link, the load-
balancer executes an algorithm. If the packet or packet fragment
contains a transport-layer header, the load balancing algorithm
accepts the following 5-tuple as input:
o IP Source Address.
o IP Destination Address.
o IPv4 Protocol or IPv6 Next Header.
o transport-layer source port.
o transport-layer destination port.
However, if the packet or packet fragment does not contain a
transport-layer header, the load balancing algorithm accepts only the
following 3-tuple as input:
o IP Source Address.
o IP Destination Address.
o 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 balancing.
4.5. Security Vulnerabilities
Security researchers have documented several attacks that rely on IP
fragmentation. The following are examples:
o Overlapping fragment attack [RFC1858][RFC3128] [RFC5722]
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o Resource exhaustion attacks (such as the Rose Attack)
o Attacks based on predictable fragment identification values
[RFC7739]
o Attacks based on bugs in the implementation of the fragment
reassembly algorithm
o Evasion of Network Intrusion Detection Systems (NIDS) [Ptacek1998]
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 non-zero
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 reassembly 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 for an
otherwise stateless protocol. As such, it can be exploited for
resource exhaustion attacks. An attacker can construct a series of
fragmented packets, with one fragment missing from each packet so
that the reassembly process cannot complete. 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.
An IP fragment contains an "Identification" field that, together with
the IP Source Address and Destination Address of a packet, identifies
fragments that correspond to the same original datagram, so that they
can be reassembled together by the receiving host. Many
implementations have employed predictable values for the
Identification field, thus making it easy for an attacker to forge
malicious IP fragments that would cause the reassembly procedure for
legitimate packets to fail.
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Over the years multiple IPv4 and IPv6 implementations have been found
to have flaws in their implementation of the IP fragment reassembly
algorithm, typically resulting in buffer overflows. These buffer
overflows have been exploitable for denial of service and remote code
execution attacks.
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 by e.g.
Computing all possible outcomes of the fragment reassembly process,
at the expense of increased processing requirements.
4.6. Blackholing Due to ICMP Loss
As stated above, an upper-layer protocol requires access the PMTU
estimate if it:
o Does not rely on IP fragmentation.
o Relies on IP source fragmentation only (i.e., fragmentation at the
source node).
In order to satisfy this requirement, the upper-layer protocol can:
o Estimate the PMTU to be equal to the IPv4 or IPv6 minimum link
MTU.
o Access the estimate that PMTUD produced.
o Execute PMTUD procedures itself.
o Execute PLPMTUD procedures.
PMTUD relies upon the network's ability to deliver ICMP PTB messages
to the source node. Therefore, if an upper-layer protocol relies on
PMTUD, it also relies on the network's ability to deliver ICMP PTB
messages to the source node.
According to [RFC4890], ICMP 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 causes PMTUD to perform less
efficiently, but does not cause it to fail completely. When the
conditions contributing to transient loss abate, the network regains
its ability to deliver ICMP PTB messages and PMTUD regains its
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ability to function. Section 4.6.1 of this document describes
conditions that lead to transient loss of ICMP PTB messages.
However, persistent loss of ICMP PTB messages causes PMTUD to fail
completely. Section 4.6.2 and Section 4.6.3 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 any other source.
4.6.1. Transient Loss
The following factors can contribute to transient loss of ICMP PTB
messages:
o Network congestion.
o Packet corruption.
o Transient routing loops.
o ICMP rate limiting.
The effect of rate limiting may be severe, as RFC 4443 recommends
strict rate limiting of IPv6 traffic.
4.6.2. Incorrect Implementation of Security Policy
Incorrect implementation of security policy can cause persistent loss
of ICMP PTB messages.
Assume that a Customer Premise Equipment (CPE) router implements the
following zone-based security policy:
o Allow any traffic to flow from the inside zone to the outside
zone.
o 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 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
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allows the ICMP PTB to flow from the outside zone to the inside zone.
If not, the implementation discards the ICMP PTB message.
When a 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 is implemented incorrectly on
many consumer CPE routers.
4.6.3. Persistant 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. Sadly, 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
updates it PMTU estimate.
4.7. Blackholing Due To Filtering
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 recent study [Huston] confirmed this finding. It reported
that 37% of sampled endpoints used IPv6-capable DNS resolvers that
were incapable of receiving a fragmented IPv6 response.
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It is difficult to determine why network operators drop fragments.
Possible causes follow:
o Hardware inability to process fragmented packets.
o Failure to change a vendor defaults.
o Unintentional misconfiguration.
o Intentional configuration (e.g., network operators consciously
chooses to drop IPv6 fragments in order to address the issues
raised in Section 4.1 through Section 4.6, above.)
5. Alternatives to IP Fragmentation
5.1. Transport Layer Solutions
The Transport Control Protocol (TCP) [RFC0793]) 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, 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:
o Manual configuration
o PMTUD
o PLPMTUD
For IPv6 nodes, manual configuration is always applicable. If the
MSS is manually configured to 1220 bytes and the packet does not
contain extension headers, the IP layer will never produce a packet
whose length is greater than the IPv6 minimum link MTU (1280 bytes).
However, manual configuration prevents TCP from taking advantage of
larger link MTU's.
RFC 8200 strongly recommends that IPv6 nodes implement PMTUD, in
order to discover and take advantage of path MTUs greater than 1280
bytes. However, as mentioned in Section 2.1, PMTUD relies upon the
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network's ability to deliver ICMP PTB messages. Therefore, PMTUD is
applicable only in environments where the risk of ICMP PTB loss is
acceptable.
By contrast, PLPMTUD does not rely upon the network's ability to
deliver ICMP PTB messages. However, in many loss-based TCP
congestion control algorithms, the dropping of a packet may cause the
TCP control algorithm to drop the congestion control window, or even
re-start with the entire slow start process. For high capacity, long
round-trip time, large volume TCP streams, the deliberate probing
with large packets and the consequent packet drop may impose too
harsh a penalty on total TCP throughput for it to be a viable
approach. [RFC4821] defines PLPMTUD procedures for TCP.
While TCP will never 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. 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) [RFC4340] and the
Stream Control Protocol (SCP) [RFC4960] 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. Both DCCP and SCP offer manual
configuration, PMTUD and PLPMTUD as mechanisms for managing that
maximum size. [I-D.fairhurst-tsvwg-datagram-plpmtud] proposes
PLPMTUD procedures for DCCP and SCP.
Currently, User Data Protocol (UDP) [RFC0768] lacks a fragmentation
mechanism of its own and relies on IP fragmentation. However,
[I-D.ietf-tsvwg-udp-options] proposes a fragmentation mechanism for
UDP.
5.2. Application Layer Solutions
[RFC8085] 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, [RFC8085] 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
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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 [RFC1122]). 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, even though the IPv4 minimum link MTU is 68 bytes.
This advice applies equally to application that run directly over IP.
6. Applications That Rely on IPv6 Fragmentation
The following applications rely on IPv6 fragmentation:
o DNS [RFC1035]
o OSPFv3 [RFC5340]
o 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 IPv6
fragmentation may exist. They are not specifically considered in the
context of this document.
6.1. DNS
DNS relies on UDP for efficiency, and the consequence is the use of
IP fragmentation for large responses, as permitted by the DNS EDNS(0)
options in the query. It is possible to mitigate the issue of
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fragmentation-based packet loss by having queries use smaller EDNS(0)
UDP buffer sizes, but then the operational issue of the partial level
of support for DNS over TCP over IPv6 becomes a limiting factor of
the efficacy of this approach in an IPv6 context [Damas].
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 adoption of DNSSEC
administrative practices that attempt to keep DNS response as short
as possible.
6.2. OSPFv3
OSPFv3 implementations can emit messages large enough to cause IPv6
fragmentation. However, in keeping with the recommendations of
RFC8200, and in order to optimize performance, most OSPFv3
implementations restrict their maximum message size to the IPv6
minimum link MTU.
6.3. Packet-in-Packet Encapsulations
In this document, packet-in-packet encapsulations include IP-in-IP
[RFC2003], Generic Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP
[RFC8086] and Generic Packet Tunneling in IPv6 [RFC2473]. [RFC4459]
describes fragmentation issues associated with all of the above-
mentioned encapsulations.
The fragmentation strategy described for GRE in [RFC7588] has been
deployed for all of the above-mentioned encapsulations. This
strategy does not rely on IPv6 fragmentation except in one corner
case. (see Section 3.3.2.2 of RFC 7588 and Section 7.1 of RFC 2473).
Section 3.3 of [RFC7676] further describes this corner case.
7. Recommendations
7.1. For Application Developers
Application developers SHOULD NOT develop applications that rely on
IPv6 fragmentation.
Application-layer protocols then depend upon IPv6 fragmentation
SHOULD be updated to break that dependency.
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7.2. For Network Operators
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 Section 4.6, filtering ICMPv6 PTB packets
causes PMTUD to fail. 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.
Many upper-layer protocols rely on PMTUD.
8. IANA Considerations
This document makes no request of IANA.
9. Security Considerations
This document mitigates some of the security considerations
associated with IP fragmentation by discouraging the use of IP
fragmentation. 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.
10. Acknowledgements
Thanks to Mikael Abrahamsson, Lorenzo Colitti, Mike Heard, Tom
Herbert, Tatuya Jinmei, Paolo Lucente, Eric Nygren, and Joe Touch for
their comments.
11. References
11.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
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[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
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11.2. Informative References
[Damas] Damas, J. and G. Huston, "Measuring ATR", April 2018,
<http://www.potaroo.net/ispcol/2018-04/atr.html>.
[Huston] Huston, G., "IPv6, Large UDP Packets and the DNS
(http://www.potaroo.net/ispcol/2017-08/xtn-hdrs.html)",
August 2017.
[I-D.fairhurst-tsvwg-datagram-plpmtud]
Fairhurst, G., Jones, T., Tuexen, M., and I. Ruengeler,
"Packetization Layer Path MTU Discovery for Datagram
Transports", draft-fairhurst-tsvwg-datagram-plpmtud-02
(work in progress), December 2017.
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-05 (work in progress), July 2018.
[Ptacek1998]
Ptacek, T. and T. Newsham, "Insertion, Evasion and Denial
of Service: Eluding Network Intrusion Detection", 1998,
<http://www.aciri.org/vern/Ptacek-Newsham-Evasion-98.ps>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
DOI 10.17487/RFC1858, October 1995,
<https://www.rfc-editor.org/info/rfc1858>.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
DOI 10.17487/RFC2003, October 1996,
<https://www.rfc-editor.org/info/rfc2003>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <https://www.rfc-editor.org/info/rfc2473>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
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[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/info/rfc2784>.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128,
DOI 10.17487/RFC3128, June 2001,
<https://www.rfc-editor.org/info/rfc3128>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
2006, <https://www.rfc-editor.org/info/rfc4459>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<https://www.rfc-editor.org/info/rfc4890>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, DOI 10.17487/RFC5722, December 2009,
<https://www.rfc-editor.org/info/rfc5722>.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>.
[RFC7588] Bonica, R., Pignataro, C., and J. Touch, "A Widely
Deployed Solution to the Generic Routing Encapsulation
(GRE) Fragmentation Problem", RFC 7588,
DOI 10.17487/RFC7588, July 2015,
<https://www.rfc-editor.org/info/rfc7588>.
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[RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
for Generic Routing Encapsulation (GRE)", RFC 7676,
DOI 10.17487/RFC7676, October 2015,
<https://www.rfc-editor.org/info/rfc7676>.
[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <https://www.rfc-editor.org/info/rfc7739>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/info/rfc7872>.
[RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
March 2017, <https://www.rfc-editor.org/info/rfc8086>.
Appendix A. Contributors' Address
Authors' Addresses
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20171
USA
Email: rbonica@juniper.net
Fred Baker
Unaffiliated
Santa Barbara, California 93117
USA
Email: FredBaker.IETF@gmail.com
Geoff Huston
APNIC
6 Cordelia St
Brisbane, 4101 QLD
Australia
Email: gih@apnic.net
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Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, California 94070
USA
Email: bob.hinden@gmail.com
Ole Troan
Cisco
Philip Pedersens vei 1
N-1366 Lysaker
Norway
Email: ot@cisco.com
Fernando Gont
SI6 Networks
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires
Argentina
Email: fgont@si6networks.com
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