Transport Area Working Group L. Eggert
Internet-Draft Nokia
Intended status: Best Current G. Fairhurst
Practice University of Aberdeen
Expires: May 22, 2008 November 19, 2007
UDP Usage Guidelines for Application Designers
draft-ietf-tsvwg-udp-guidelines-04
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Copyright (C) The IETF Trust (2007).
Abstract
The User Datagram Protocol (UDP) provides a minimal, message-passing
transport that has no inherent congestion control mechanisms.
Because congestion control is critical to the stable operation of the
Internet, applications and upper-layer protocols that choose to use
UDP as an Internet transport must employ mechanisms to prevent
congestion collapse and establish some degree of fairness with
concurrent traffic. This document provides guidelines on the use of
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UDP for the designers of such applications and upper-layer protocols.
Congestion control guidelines are a primary focus, but the document
also provides guidance on other topics, including message sizes,
reliability, checksums and middlebox traversal.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. UDP Usage Guidelines . . . . . . . . . . . . . . . . . . . . . 4
3.1. Congestion Control Guidelines . . . . . . . . . . . . . . 5
3.2. Message Size Guidelines . . . . . . . . . . . . . . . . . 10
3.3. Reliability Guidelines . . . . . . . . . . . . . . . . . . 10
3.4. Checksum Guidelines . . . . . . . . . . . . . . . . . . . 11
3.5. Middlebox Traversal Guidelines . . . . . . . . . . . . . . 13
3.6. Programming Guidelines . . . . . . . . . . . . . . . . . . 14
3.7. ICMP Guidelines . . . . . . . . . . . . . . . . . . . . . 16
4. Security Considerations . . . . . . . . . . . . . . . . . . . 16
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
The User Datagram Protocol (UDP) [RFC0768] provides a minimal,
unreliable, best-effort, message-passing transport to applications
and upper-layer protocols (both simply called "applications" in the
remainder of this document). Compared to other transport protocols,
UDP and its UDP-Lite variant [RFC3828] are unique in that they do not
establish end-to-end connections between communicating end systems.
UDP communication consequently does not incur connection
establishment and teardown overheads and there is minimal associated
end system state. Because of these characteristics, UDP can offer a
very efficient communication transport to some applications.
A second unique characteristic of UDP is that it provides no inherent
congestion control mechanisms. On many platforms, applications can
send UDP messages at the line rate of the link interface, which is
often much greater than the available path capacity, and doing so
contributes to congestion along the path. [RFC2914] describes the
best current practice for congestion control in the Internet. It
identifies two major reasons why congestion control mechanisms are
critical for the stable operation of the Internet:
1. The prevention of congestion collapse, i.e., a state where an
increase in network load results in a decrease in useful work
done by the network.
2. The establishment of a degree of fairness, i.e., allowing
multiple flows to share the capacity of a path reasonably
equitably.
Because UDP itself provides no congestion control mechanisms, it is
up to the applications that use UDP for Internet communication to
employ suitable mechanisms to prevent congestion collapse and
establish a degree of fairness. [RFC2309] discusses the dangers of
congestion-unresponsive flows and states that "all UDP-based
streaming applications should incorporate effective congestion
avoidance mechanisms." This is an important requirement, even for
applications that do not use UDP for streaming. For example, an
application that generates five 1500-byte UDP messages in one second
can already exceed the capacity of a 56 Kb/s path. For applications
that can operate at higher, potentially unbounded data rates,
congestion control becomes vital to prevent congestion collapse and
establish some degree of fairness. Section 3 describes a number of
simple guidelines for the designers of such applications.
A UDP message is carried in a single IP packet and is hence limited
to a maximum payload of 65,507 bytes for IPv4 and 65,487 bytes for
IPv6. The transmission of large IP packets usually requires IP
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fragmentation. At least for IPv4, fragmentation decreases
communication reliability and efficiency and should be avoided. One
reason for this decrease in reliability is that many NATs and
firewalls do not forward IPv4 fragments; other reasons are documented
in [RFC4963]. Some of the guidelines in Section 3 describe how
applications should determine appropriate message sizes.
This document provides guidelines to designers of applications that
use UDP for unicast transmission. A special class of applications
uses UDP for IP multicast transmissions. Congestion control, flow
control or reliability for multicast transmissions is more difficult
to establish than for unicast transmissions, because a single sender
may transmit to multiple receivers across potentially very
heterogeneous paths at the same time. Designing multicast
applications requires expertise that goes beyond the simple
guidelines given in this document. The IETF has defined a reliable
multicast framework [RFC3048] and several building blocks to aid the
designers of multicast applications, such as [RFC3738] or [RFC4654].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
3. UDP Usage Guidelines
Internet paths can have widely varying characteristics, including
transmission delays, available bandwidths, congestion levels,
reordering probabilities, supported message sizes or loss rates.
Furthermore, the same Internet path can have very different
conditions over time. Consequently, applications that may be used on
the Internet MUST NOT make assumptions about specific path
characteristics. They MUST instead use mechanisms that let them
operate safely under very different path conditions. Typically, this
requires conservatively probing the Internet path to establish a
transmission behavior that it can sustain and that is reasonably fair
to other traffic sharing the path.
These mechanisms are difficult to implement correctly. For most
applications, the use of one of the existing IETF transport protocols
is the simplest method of acquiring the required mechanisms.
Consequently, the RECOMMENDED alternative to the UDP usage described
in the remainder of this section is the use of an IETF transport
protocol such as TCP [RFC0793], SCTP [RFC4960] and SCTP-PR [RFC3758],
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or DCCP [RFC4340] with its different congestion control types
[RFC4341][RFC4342][I-D.ietf-dccp-ccid4].
If used correctly, these more fully-featured transport protocols are
not as "heavyweight" as often claimed. For example, TCP's "Nagle"
algorithm [RFC0896] can be disabled, improving communication latency
at the expense of more frequent - but still congestion-controlled -
packet transmissions. Another example is the TCP SYN Cookie
mechanism [RFC4987], which is available on many platforms. TCP with
SYN Cookies does not require a server to maintain per-connection
state until the connection is established. TCP also requires the end
that closes a connection to maintain the TIME-WAIT state that
prevents delayed segments from one connection instance to interfere
with a later one. Applications that are aware of and designed for
this behavior can shift maintenance of the TIME-WAIT state to
conserve resources by controlling which end closes a TCP connection
[FABER]. Finally, TCP's built-in capacity-probing and awareness of
the maximum transmission unit supported by the path (PMTU) results in
efficient data transmission that quickly compensates for the initial
connection setup delay, for transfers that exchange more than a few
messages.
3.1. Congestion Control Guidelines
If an application or upper-layer protocol chooses not to use a
congestion-controlled transport protocol, it SHOULD control the rate
at which it sends UDP messages to a destination host, in order to
fulfill the requirements of [RFC2914]. It is important to stress
that an application SHOULD perform congestion control over all UDP
traffic it sends to a destination, independently from how it
generates this traffic. For example, an application that forks
multiple worker processes or otherwise uses multiple sockets to
generate UDP messages SHOULD perform congestion control over the
aggregate traffic.
The remainder of this section discusses several approaches for this
purpose. Not all approaches discussed below are appropriate for all
UDP-transmitting applications. Section 3.1.1 discusses congestion
control options for applications that perform bulk transfers over
UDP. Such applications can employ schemes that sample the path over
several subsequent RTTs during which data is exchanged, in order to
determine a sending rate that the path at its current load can
support. Other applications only exchange a few UDP messages with a
destination. Section 3.1.2 discusses congestion control options for
such "low data-volume" applications. Because they typically do not
transmit enough data to iteratively sample the path to determine a
safe sending rate, they need to employ different kinds of congestion
control mechanisms. Finally, Section 3.1.3 discusses congestion
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control considerations when UDP is used as a tunneling protocol.
It is important to note that congestion control should not be viewed
as an add-on to a finished application. Many of the mechanisms
discussed in the guidelines below require application support to
operate correctly. Application designers need to consider congestion
control throughout the design of their application, similar to how
they consider security aspects throughout the design process.
Finally, in the past, the IETF has investigated integrated congestion
control mechanisms that act on the traffic aggregate between two
hosts, i.e., across all communication sessions active at a given time
independent of specific transport protocols, such as the Congestion
Manager [RFC3124]. Such mechanisms have failed to see deployment,
but would otherwise also fulfill the congestion control requirements
in [RFC2914], because they provide congestion control for UDP
sessions.
3.1.1. Bulk Transfer Applications
Applications that perform bulk transmission of data to a peer over
UDP, i.e., applications that exchange more than a small number of
messages per RTT, SHOULD implement TCP-Friendly Rate Control (TFRC)
[RFC3448], window-based, TCP-like congestion control, or otherwise
ensure that the application complies with the congestion control
principles.
TFRC has been designed to provide both congestion control and
fairness in a way that is compatible with the IETF's other transport
protocols. TFRC is currently being updated
[I-D.ietf-dccp-rfc3448bis], and application designers SHOULD always
evaluate whether the latest published specification fits their needs.
If an application implements TFRC, it need not follow the remaining
guidelines in Section 3.1, because TFRC already addresses them, but
SHOULD still follow the remaining guidelines in the subsequent
subsections of Section 3.
Bulk transfer applications that choose not to implement TFRC or TCP-
like windowing SHOULD implement a congestion control scheme that
results in bandwidth use that competes fairly with TCP within an
order of magnitude. [RFC3551] suggests that applications SHOULD
monitor the packet loss rate to ensure that it is within acceptable
parameters. Packet loss is considered acceptable if a TCP flow
across the same network path under the same network conditions would
achieve an average throughput, measured on a reasonable timescale,
that is not less than that of the UDP flow. The comparison to TCP
cannot be specified exactly, but is intended as an "order-of-
magnitude" comparison in timescale and throughput.
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Finally, some bulk transfer applications chose not to implement any
congestion control mechanism and instead rely on transmitting across
reserved path capacity. This might be an acceptable choice for a
subset of restricted networking environments, but is by no means a
safe practice for operation in the Internet. When the UDP traffic of
such applications leaks out on unprovisioned Internet paths, it can
significantly degrade the performance of other traffic sharing the
path and even result in congestion collapse. Applications that
support an uncontrolled or unadaptive transmission behavior SHOULD
NOT do so by default and SHOULD instead require users to explicitly
enable this mode of operation.
3.1.2. Low Data-Volume Applications
When applications that exchange only a small number of messages with
a destination at any time implement TFRC or one of the other
congestion control schemes in Section 3.1.1, the network sees little
benefit, because those mechanisms perform congestion control in a way
that is only effective for longer transmissions.
Applications that exchange only a small number of messages with a
destination at any time SHOULD still control their transmission
behavior by not sending more than one UDP message per round-trip time
(RTT) to a destination. Similar to the recommendation in [RFC1536],
an application SHOULD maintain an estimate of the RTT for any
destination with which it communicates. Applications SHOULD
implement the algorithm specified in [RFC2988] to compute a smoothed
RTT (SRTT) estimate. A lost response from the peer SHOULD be treated
as a very large RTT sample, instead of being ignored, in order to
cause a sufficiently large (exponential) back-off. When implementing
this scheme, applications need to choose a sensible initial value for
the RTT. This value SHOULD generally be as conservative as possible
for the given application. TCP uses an initial value of 3 seconds
[RFC2988], which is also RECOMMENDED as an initial value for UDP
applications. SIP [RFC3261] and GIST [I-D.ietf-nsis-ntlp] use an
initial value of 500 ms, and initial timeouts that are shorter than
this are likely problematic in many cases. It is also important to
note that the initial timeout is not the maximum possible timeout -
the RECOMMENDED algorithm in [RFC2988] yields timeout values after a
series of losses that are much longer than the initial value.
Some applications cannot maintain a reliable RTT estimate for a
destination. The first case is applications that exchange too few
messages with a peer to establish a statistically accurate RTT
estimate. Such applications MAY use a pre-determined transmission
interval that is exponentially backed-off when packets are lost. TCP
uses an initial value of 3 seconds [RFC2988], which is also
RECOMMENDED as an initial value for UDP applications. SIP [RFC3261]
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and GIST [I-D.ietf-nsis-ntlp] use an interval of 500 ms, and shorter
values are likely problematic in many cases. As in the previous
case, note that the initial timeout is not the maximum possible
timeout.
A second class of applications cannot maintain an RTT estimate for a
destination, because the destination does not send return traffic.
Such applications SHOULD NOT send more than one UDP message every 3
seconds, and SHOULD use an even less aggressive rate when possible.
The 3-second interval was chosen based on TCP's retransmission
timeout when the RTT is unknown [RFC2988], and shorter values are
likely problematic in many cases. Note that the initial timeout
interval must be more conservative than in the two previous cases,
because the lack of return traffic prevents the detection of packet
loss, i.e., congestion events, and the application therefore cannot
perform exponential back-off to reduce load.
Applications that communicate bidirectionally SHOULD employ
congestion control for both directions of the communication. For
example, for a client-server, request-response-style application,
clients SHOULD congestion control their request transmission to a
server, and the server SHOULD congestion-control its responses to the
clients. Congestion in the forward and reverse direction is
uncorrelated and an application SHOULD independently detect and
respond to congestion along both directions.
3.1.3. UDP Tunnels
One increasingly popular use of UDP is as a tunneling protocol, where
a tunnel endpoint encapsulates the packets of another protocol inside
UDP messages and transmits them to another tunnel endpoint, which
decapsulates the UDP messages and forwards the original packets
contained in the payload. Tunnels establish virtual links that
appear to directly connect locations that are distant in the physical
Internet topology, and can be used to create virtual (private)
networks. Using UDP as a tunneling protocol is attractive when the
payload protocol is not supported by middleboxes that may exist along
the path, because many middleboxes support UDP transmissions.
Well-implemented tunnels are generally invisible to the endpoints
that happen to transmit over a path that includes tunneled links. On
the other hand, to the routers along the path of a UDP tunnel, i.e.,
the routers between the two tunnel endpoints, the traffic that a UDP
tunnel generates is a regular UDP flow, and the encapsulator and
decapsulator appear as regular UDP-sending and -receiving
applications. Because other flows can share the path with one or
more UDP tunnels, congestion control needs to be considered.
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Two factors determine whether a UDP tunnel needs to employ specific
congestion control mechanisms. First, whether the tunneling scheme
generates UDP traffic at a volume that corresponds to the volume of
payload traffic carried within the tunnel. Second, whether the
payload traffic is IP-based. This results in these possible cases:
1. Tunnel generates UDP traffic at a volume that corresponds to the
volume of payload traffic, and the payload traffic is IP-based.
This is arguably the most common case for Internet tunnels. In
this case, the UDP tunnel SHOULD NOT employ its own congestion
control mechanism, because congestion losses of tunneled traffic
will already trigger an appropriate congestion response at the
original senders of the tunneled traffic.
Note that this guideline is built on the assumption that most IP-
based communication is congestion-controlled. If a UDP tunnel is
used for IP-based traffic that is known to not be congestion-
controlled, the next set of guidelines applies:
2. Tunnel generates UDP traffic at a volume that corresponds to the
volume of payload traffic, and the payload traffic is not known
to be IP-based (or is known to be IP-based, but not congestion-
controlled).
This is the case, for example, when link-layer protocols are
encapsulated within UDP. Because it is not known that congestion
losses of tunneled non-IP traffic will trigger an appropriate
congestion response at the senders, the UDP tunnel SHOULD employ
an appropriate congestion control mechanism. Because tunnels are
usually bulk-transfer applications as far as the intermediate
routers are concerned, the guidelines in Section 3.1.1 apply.
3. Tunnel generates UDP traffic at a volume that does not correspond
to the volume of payload traffic, independent of whether the
payload traffic is IP-based or not.
Examples of this class include UDP tunnels that send at a
constant rate, increase their transmission rates under loss, for
example, due to increasing redundancy when forward-error-
correction is used, or are otherwise constrained in their
transmission behavior. These specialized uses of UDP for
tunneling go beyond the scope of the general guidelines given in
this document. The implementer of such specialized tunnels
SHOULD carefully consider congestion control in the design of
their tunneling mechanism.
Designing tunneling mechanism requires significantly more expertise
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than needed for many other UDP applications, because tunnels
virtualize lower-layer components of the Internet, and the
virtualized components need to correctly interact with the
infrastructure at that layer. This document only touches upon the
congestion control considerations for implementing UDP tunnels; a
discussion of other required tunneling behavior is out of scope.
3.2. Message Size Guidelines
Because IPv4 fragmentation lowers the efficiency and reliability of
Internet communication [RFC4963], an application SHOULD NOT send UDP
messages that result in IPv4 packets that exceed the MTU of 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 itself [RFC1191][RFC1981][RFC4821] to determine whether
the path to a destination will support its desired message size
without fragmentation.
Applications that choose to not adapt their transmit message size
SHOULD NOT send UDP messages that would result in IP datagrams that
exceed the effective PMTU. In the absence of actual knowledge of the
minimum MTU along the path, the effective PMTU depends upon the IP
version used for transmission. It is the smaller of 576 bytes and
the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for IPv6
[RFC2460]. 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 messages is inefficient over paths that support a
larger PMTU, which is a second reason to implement PMTU discovery.
To determine an appropriate UDP payload size, applications MUST
subtract the size of the IP header (which includes any IPv4 optional
headers or IPv6 extension headers) as well as the length of the UDP
header (8 bytes) from the PMTU size. This size, known as the MMS_S,
can be obtained from the TCP/IP stack [RFC1122].
Applications that do not send messages that exceed the effective PMTU
of IPv4 or IPv6 need not implement any of the above mechanisms. Note
that the presence of tunnels can cause fragmentation even when
applications send messages that do not exceed the effective PMTU, so
implementing PMTU discovery will still be beneficial in some cases.
3.3. Reliability Guidelines
Application designers are generally aware that UDP does not provide
any reliability, e.g., it does not retransmit any lost packets.
Often, this is a main reason to consider UDP as a transport.
Applications that do require reliable message delivery MUST implement
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an appropriate mechanism themselves.
UDP also does not protect against message duplication, i.e., an
application may receive multiple copies of the same message.
Application designers SHOULD verify that their application handles
message duplication gracefully, and may consequently need to
implement mechanisms to detect duplicates. Even if message reception
triggers idempotent operations, applications may want to suppress
duplicate messages to reduce load.
In addition, the Internet can significantly delay some packets with
respect to others, e.g., due to routing transients, intermittent
connectivity, or mobility. This can cause message reordering, where
UDP messages arrive at the receiver in an order different from the
transmission order. Applications that require ordered delivery MUST
reestablish message ordering themselves.
Finally, it is important to note that delay spikes can be very large.
This can cause reordered packets to arrive many seconds after they
were sent. The Internet protocol suite defines the Maximum Segment
Lifetime (MSL) for TCP segments as 2 minutes [RFC0793]; this value
applies to all IP datagrams, and hence also to UDP. The MSL is the
maximum delay a packet should experience. Applications SHOULD be
robust to the reception of delayed or duplicate packets that are
received within this two-minute interval.
Applications that require reliable and ordered message delivery
SHOULD choose an IETF standard transport protocol that provides these
features. If this is not possible, it will need to implement a set
of appropriate mechanisms itself.
3.4. Checksum Guidelines
The UDP header includes an optional, 16-bit ones-complement checksum
that provides an integrity check. This results in a relatively weak
protection from a coding point of view [RFC3819] and application
developers SHOULD implement additional checks where data integrity is
important, e.g., through a Cyclic Redundancy Check (CRC) included
with the data to verify the integrity of an entire object/file sent
over UDP service.
The UDP checksum provides assurance that the payload was not
corrupted in transit. It also allows the receiver to verify that it
was the intended destination of the packet, because it covers the IP
addresses, port numbers and protocol number, and it verifies that the
packet is not truncated or padded, because it covers the size field.
It therefore protects an application against receiving corrupted
payload data in place of, or in addition to, the data that was sent.
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Applications SHOULD enable UDP checksums, although [RFC0768] permits
the option to disable their use. Applications that choose to disable
UDP checksums when transmitting over IPv4 therefore MUST NOT make
assumptions regarding the correctness of received data and MUST
behave correctly when a message is received that was originally sent
to a different destination or is otherwise corrupted. The use of the
UDP checksum is MANDATORY when applications transmit UDP over IPv6
[RFC2460].
3.4.1. UDP-Lite
A special class of applications can derive benefit from having
partially damaged payloads delivered, rather than discarded, when
using paths that include error-prone links. Such applications can
tolerate payload corruption and MAY choose to use the Lightweight
User Datagram Protocol (UDP-Lite) [RFC3828] variant of UDP instead of
basic UDP. Applications that choose to use UDP-Lite instead of UDP
MUST still follow the congestion control and other guidelines
described for use with UDP in Section 3.
UDP-Lite changes the semantics of the UDP "payload length" field to
that of a "checksum coverage length" field. Otherwise, UDP-Lite is
semantically identical to UDP. The interface of UDP-Lite differs
from that of UDP by the addition of a single (socket) option that
communicates a checksum coverage length value: at the sender, this
specifies the intended checksum coverage, with the remaining
unprotected part of the payload called the "error insensitive part".
If required, an application may dynamically modify this length value,
e.g., to offer greater protection to some messages. UDP-Lite always
verifies that a packet was delivered to the intended destination,
i.e., always verifies the header fields. Errors in the insensitive
part will not cause a UDP message to be discarded by the destination.
Applications using UDP-Lite therefore MUST NOT make assumptions
regarding the correctness of the data received in the insensitive
part of the UDP-Lite payload.
The sending application SHOULD select the minimum checksum coverage
to include all sensitive protocol headers. For example, applications
that use the Real-Time Protocol (RTP) [RFC3550] will likely want to
protect the RTP header against corruption. Applications, where
appropriate, MUST also introduce their own appropriate validity
checks for protocol information carried in the insensitive part of
the UDP-Lite payload (e.g., internal CRCs).
The receiver MUST set a minimum coverage threshold for incoming
packets that is not smaller than the smallest coverage used by the
sender. This may be a fixed value, or may be negotiated by an
application. UDP-Lite does not provide mechanisms to negotiate the
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checksum coverage between the sender and receiver.
Applications may still experience packet loss, rather than
corruption, when using UDP-Lite. The enhancements offered by UDP-
Lite rely upon a link being able to intercept the UDP-Lite header to
correctly identify the partial-coverage required. When tunnels
and/or encryption are used, this can result in UDP-Lite messages
being treated the same as UDP messages, i.e., result in packet loss.
Use of IP fragmentation can also prevent special treatment for UDP-
Lite messages, and is another reason why applications SHOULD avoid IP
fragmentation Section 3.2.
3.5. Middlebox Traversal Guidelines
Network address translators (NATs) and firewalls are examples of
intermediary devices ("middleboxes") that can exist along an end-to-
end path. A middlebox typically performs a function that requires it
to maintain per-flow state. For connection-oriented protocols, such
as TCP, middleboxes snoop and parse the connection-management traffic
and create and destroy per-flow state accordingly. For a
connectionless protocol such as UDP, this approach is not possible.
Consequently, middleboxes may create per-flow state when they see a
packet that indicates a new flow, and destroy the state after some
period of time during which no packets belonging to the same flow
have arrived.
Depending on the specific function that the middlebox performs, this
behavior can introduce a time-dependency that restricts the kinds of
UDP traffic exchanges that will be successful across the middlebox.
For example, NATs and firewalls typically define the partial path on
one side of them to be interior to the domain they serve, whereas the
partial path on their other side is defined to be exterior to that
domain. Per-flow state is typically created when the first packet
crosses from the interior to the exterior, and while the state is
present, NATs and firewalls will forward return traffic. Return
traffic arriving after the per-flow state has timed out is dropped,
as is other traffic arriving from the exterior.
Many applications that use UDP for communication operate across
middleboxes without needing to employ additional mechanisms. One
example is the DNS, which has a strict request-response communication
pattern that typically completes within seconds.
Other applications may experience communication failures when
middleboxes destroy the per-flow state associated with an application
session during periods when the application does not exchange any UDP
traffic. Applications SHOULD be able to gracefully handle such
communication failures and implement mechanisms to re-establish
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application-layer sessions and state.
For some applications, such as media transmissions, this re-
synchronization is highly undesirable, because it can cause user-
perceivable playback artifacts. Such specialized applications MAY
send periodic keep-alive messages to attempt to refresh middlebox
state. It is important to note that keep-alive messages are NOT
RECOMMENDED for general use - they are unnecessary for many
applications and can consume significant amounts of system and
network resources.
An application that needs to employ keep-alives to deliver useful
service in the presence of middleboxes SHOULD NOT transmit them more
frequently than once every 15 seconds and SHOULD use longer intervals
when possible. No common timeout has been specified for per-flow UDP
state for arbitrary middleboxes. For NATs, [RFC4787] requires a
state timeout of 2 minutes or longer. However, empirical evidence
suggests that a significant fraction of the deployed middleboxes
unfortunately uses shorter timeouts. The timeout of 15 seconds
originates with the Interactive Connectivity Establishment (ICE)
protocol [I-D.ietf-mmusic-ice]. Applications that operate in more
controlled network environments SHOULD investigate whether the
environment they operate in allows them to use longer intervals, or
whether it offers mechanisms to explicitly control middlebox state
timeout durations, for example, using MIDCOM [RFC3303], NSIS
[I-D.ietf-nsis-nslp-natfw], STUN
[I-D.wing-behave-nat-control-stun-usage] or UPnP [UPNP].
It is important to note that sending keep-alives is not a substitute
for implementing robust connection handling. Like all UDP messages,
keep-alives can be delayed or dropped, causing middlebox state to
time out. In addition, the congestion control guidelines in
Section 3.1 cover all UDP transmissions by an application, including
the transmission of middlebox keep-alives. Congestion control may
thus lead to delays or temporary suspension of keep-alive
transmission.
3.6. Programming Guidelines
The de facto standard application programming interface (API) for
TCP/IP applications is the "sockets" interface [POSIX]. Although
this API was developed for UNIX in the early 1980s, a wide variety of
non-UNIX operating systems also implements it. The sockets API
supports both IPv4 and IPv6 [RFC3493]. The UDP sockets API differs
from that for TCP in several key ways. Because application
programmers are typically more familiar with the TCP sockets API, the
remainder of this section discusses these differences. [STEVENS]
provides usage examples of the UDP sockets API.
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UDP messages may be directly sent and received, without any
connection setup. Using the sockets API, applications can receive
packets from more than one IP source address on a single UDP socket.
Some servers use this to exchange data with more than one remote host
through a single UDP socket at the same time. When applications need
to ensure that they receive packets from a particular source address,
they MUST implement corresponding checks at the application layer or
explicitly request that the operating system filter the received
packets.
If a client/server application executes on a host with more than one
IP interface, the application MUST ensure that it sends any UDP
responses in reply to arriving UDP datagrams with an IP source
address that matches the IP destination address of the datagram that
carried the request.
A UDP receiver can receive a valid UDP datagram with a zero-length
payload. Note that this is different from a return value of zero
from a read() socket call, which for TCP indicates the end of the
connection.
Many operating systems also allow a UDP socket to be connected, i.e.,
to bind a UDP socket to a specific pair of addresses and ports. This
is similar to the corresponding TCP sockets API functionality.
However, for UDP, this is only a local operation that serves to
simplify the local send/receive functions and to filter the traffic
for the specified addresses and ports. Binding a UDP socket does not
establish a connection - UDP does not notify the remote end when a
local UDP socket is bound. Binding a socket also allows configuring
options that affect the UDP or IP layers, for example, use of the UDP
checksum or IP source routing. On some stacks, a bound socket also
allows an application to be notified when ICMP error messages are
received for its transmissions [RFC1122].
UDP provides no flow-control. This is another reason why UDP-based
applications need to be robust in the presence of packet loss. This
loss can also occur within the sending host, when an application
sends data faster than the line rate of the outbound network
interface. It can also occur on the destination, where receive calls
fail to return data when the application issues them too frequently
(i.e., when no new data has arrived) or not frequently enough (i.e.,
such that the receive buffer overflows). Robust flow control
mechanisms are difficult to implement, which is why applications that
need this functionality SHOULD consider using a full-featured
transport protocol.
When an application closes a TCP, SCTP or DCCP socket, the transport
protocol on the receiving host is required to maintain TIME-WAIT
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state. This prevents delayed packets from the closed connection
instance from being mistakenly associated with a later connection
instance that happens to reuse the same IP address and port pairs.
The UDP protocol does not implement such a mechanism. Therefore,
UDP-based applications need to robust to this case. One application
may close a socket or terminate, followed in time by another
application receiving on the same port. This later application may
then receive packets intended for the first application that were
delayed in the network.
3.7. ICMP Guidelines
Applications can utilize ICMP error messages that the UDP layer
passes up for a variety of purposes [RFC1122]. Applications SHOULD
validate the information in the ICMP message payload, e.g., that the
reported error corresponds to a UDP datagram that the application
actually sent.
Any application response to ICMP error messages SHOULD be robust to
temporary routing failures, i.e., transient ICMP "unreachable"
messages should not normally cause a communication abort.
Applications are RECOMMENDED to appropriately respond to ICMP
messages generated in response to transmitted traffic. A correct
response often requires context, such as local state about
communication instances to each destination, that although readily
available in connection-orientated transport protocols is not always
maintained by UDP-based applications.
4. Security Considerations
UDP does not provide communications security. Applications that need
to protect their communications against eavesdropping, tampering, or
message forgery SHOULD employ end-to-end security services provided
by other IETF protocols.
One option of securing UDP communications is with IPsec [RFC4301],
which can provide authentication for flows of IP packets through the
Authentication Header (AH) [RFC4302] and encryption and/or
authentication through the Encapsulating Security Payload (ESP)
[RFC4303]. Applications use the Internet Key Exchange (IKE)
[RFC4306] to configure IPsec for their sessions. Depending on how
IPsec is configured for a flow, it can authenticate or encrypt the
UDP headers as well as UDP payloads. If an application only requires
authentication, ESP with no encryption but with authentication is
often a better option than AH, because ESP can operate across
middleboxes. In order to be able to use IPsec, an application must
execute on an operating system that implements the IPsec protocol
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suite.
Although it is possible to use IPsec to secure UDP communications,
not all operating systems support IPsec or allow applications to
easily configure it for their flows. A second option of securing UDP
communications is through Datagram Transport Layer Security (DTLS)
[RFC4347]. DTLS provides communication privacy by encrypting UDP
payloads. It does not protect the UDP headers. Applications can
implement DTLS without relying on support from the operating system.
Many other options of authenticating or encrypting UDP payloads
exist, including other IETF standards, such as S/MIME [RFC3851] or
PGP [RFC4880], security frameworks such as GSS-API [RFC1964], SASL
[RFC4422] and EAP [RFC3748], as well as many non-IETF protocols. Out
of these, S/MIME and PGP are likely to better suit less immediate and
less ephemeral communications than typically the case for UDP
applications, because they generally require public-key operations
for each message.
Like congestion control mechanisms, security mechanisms are difficult
to design and implement correctly. It is hence RECOMMENDED that
applications employ well-known standard security mechanisms such as
DTLS or IPsec, rather than inventing their own.
In terms of congestion control, [RFC2309] and [RFC2914] discuss the
dangers of congestion-unresponsive flows to the Internet. This
document provides guidelines to designers of UDP-based applications
to congestion-control their transmissions, and does not raise any
additional security concerns.
5. Summary
This section summarizes the guidelines made in Section 3 and
Section 4 in a tabular format in Table 1 for easy referencing.
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+---------------------------------------------------------+---------+
| Recommendation | Section |
+---------------------------------------------------------+---------+
| MUST accommodate wide range of Internet path conditions | 3 |
| SHOULD use a full-featured transport (TCP, SCTP, DCCP) | |
| | |
| SHOULD control rate of transmission | 3.1 |
| SHOULD perform congestion control over all traffic | |
| | |
| for bulk transfers, | 3.1.1 |
| SHOULD consider implementing TFRC | |
| else, SHOULD otherwise use bandwidth similar to TCP | |
| | |
| for non-bulk transfers, | 3.1.2 |
| SHOULD measure RTT and transmit 1 message/RTT | |
| else, SHOULD send at most 1 message every 3 seconds | |
| | |
| SHOULD NOT send messages that exceed the PMTU, i.e., | 3.2 |
| SHOULD discover PMTU or send messages < minimum PMTU | |
| | |
| SHOULD handle message loss, duplication, reordering | 3.3 |
| | |
| SHOULD enable UDP checksum | 3.4 |
| else, MAY use UDP-Lite with suitable checksum coverage | 3.4.1 |
| | |
| SHOULD NOT always send middlebox keep-alives | 3.5 |
| MAY use keep-alives when needed (min. interval 15 sec) | |
| | |
| MUST check IP source address | 3.6 |
| | |
| SHOULD use standard IETF security protocols when needed | 4 |
+---------------------------------------------------------+---------+
Table 1: Summary of recommendations.
6. IANA Considerations
This document raises no IANA considerations.
7. Acknowledgments
Thanks to Paul Aitken, Mark Allman, Francois Audet, Stewart Bryant,
Remi Denis-Courmont, Wesley Eddy, Sally Floyd, Jeffrey Hutzelman,
Tero Kivinen, Philip Matthews, Joerg Ott, Colin Perkins, Carlos
Pignataro, Pasi Sarolahti, Joe Touch and Magnus Westerlund for their
comments on this document.
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The middlebox traversal guidelines in Section 3.5 incorporate ideas
from Section 5 of [I-D.ford-behave-app] by Bryan Ford, Pyda Srisuresh
and Dan Kegel.
8. References
8.1. Normative References
[POSIX] IEEE Std. 1003.1-2001, "Standard for Information
Technology - Portable Operating System Interface (POSIX)",
Open Group Technical Standard: Base Specifications Issue
6, ISO/IEC 9945:2002, December 2001.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
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[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
8.2. Informative References
[FABER] Faber, T., Touch, J., and W. Yue, "The TIME-WAIT State in
TCP and Its Effect on Busy Servers", Proc. IEEE Infocom,
March 1999.
[I-D.ford-behave-app]
Ford, B., "Application Design Guidelines for Traversal
through Network Address Translators",
draft-ford-behave-app-05 (work in progress), March 2007.
[I-D.ietf-dccp-ccid4]
Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly
Rate Control for Small Packets (TFRC-SP)",
draft-ietf-dccp-ccid4-00 (work in progress), October 2007.
[I-D.ietf-dccp-rfc3448bis]
Handley, M., "TCP Friendly Rate Control (TFRC): Protocol
Specification", draft-ietf-dccp-rfc3448bis-02 (work in
progress), July 2007.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-nsis-nslp-natfw]
Stiemerling, M., "NAT/Firewall NSIS Signaling Layer
Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-15 (work in
progress), July 2007.
[I-D.ietf-nsis-ntlp]
Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-14 (work in
progress), July 2007.
[I-D.wing-behave-nat-control-stun-usage]
Wing, D., Rosenberg, J., and H. Tschofenig, "Discovering,
Querying, and Controlling Firewalls and NATs",
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draft-wing-behave-nat-control-stun-usage-05 (work in
progress), October 2007.
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
[RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
Floyd, S., and M. Luby, "Reliable Multicast Transport
Building Blocks for One-to-Many Bulk-Data Transfer",
RFC 3048, January 2001.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, June 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
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[RFC3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate
Control (WEBRC) Building Block", RFC 3738, April 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, May 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
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Security Layer (SASL)", RFC 4422, June 2006.
[RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[STEVENS] Stevens, W., Fenner, B., and A. Rudoff, "UNIX Network
Programming, The sockets Networking API", Addison-Wesley,
2004.
[UPNP] UPnP Forum, "Internet Gateway Device (IGD) Standardized
Device Control Protocol V 1.0", November 2001.
Authors' Addresses
Lars Eggert
Nokia Research Center
P.O. Box 407
Nokia Group 00045
Finland
Phone: +358 50 48 24461
Email: lars.eggert@nokia.com
URI: http://research.nokia.com/people/lars_eggert/
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Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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