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Keywords: [--------], DCCP-Request Ports
Network Working Group G. Fairhurst
Request for Comments: 5595 University of Aberdeen
Updates: 4340 September 2009
Category: Standards Track
The Datagram Congestion Control Protocol (DCCP) Service Codes
Abstract
This document describes the usage of Service Codes by the Datagram
Congestion Control Protocol, RFC 4340. It motivates the setting of a
Service Code by applications. Service Codes provide a method to
identify the intended service/application to process a DCCP
connection request. This provides improved flexibility in the use
and assignment of port numbers for connection multiplexing. The use
of a DCCP Service Code can also enable more explicit coordination of
services with middleboxes (e.g., network address translators and
firewalls). This document updates the specification provided in RFC
4340.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright and License Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
Fairhurst Standards Track [Page 1]
RFC 5595 DCCP Service Codes September 2009
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
1.1. History ....................................................3
1.2. Conventions Used in This Document ..........................6
2. An Architecture for Service Codes ...............................6
2.1. IANA Port Numbers ..........................................6
2.2. DCCP Service Code Values ...................................7
2.2.1. New Versions of Applications or Protocols ...........8
2.3. Service Code Registry ......................................8
2.4. Zero Service Code ..........................................9
2.5. Invalid Service Code .......................................9
2.6. SDP for Describing Service Codes ...........................9
2.7. A Method to Hash the Service Code to a Dynamic Port ........9
3. Use of the DCCP Service Code ...................................10
3.1. Setting Service Codes at the Client .......................11
3.2. Using Service Codes in the Network ........................11
3.3. Using Service Codes at the Server .........................12
3.3.1. Reception of a DCCP-Request ........................13
3.3.2. Multiple Associations of a Service Code
with Ports .........................................14
3.3.3. Automatically Launching a Server ...................14
4. Security Considerations ........................................14
4.1. Server Port Number Reuse ..................................15
4.2. Association of Applications with Service Codes ............15
4.3. Interactions with IPsec ...................................15
5. IANA Considerations ............................................16
6. Acknowledgments ................................................16
7. References .....................................................17
7.1. Normative References ......................................17
7.2. Informative References ....................................17
Fairhurst Standards Track [Page 2]
RFC 5595 DCCP Service Codes September 2009
1. Introduction
DCCP specifies a Service Code as a 4-byte value (32 bits) that
describes the application-level service to which a client application
wishes to connect ([RFC4340], Section 8.1.2). A Service Code
identifies the protocol (or the standard profile, e.g., [RTP-DCCP])
to be used at the application layer. It is not intended to be used
to specify a variant of an application or a specific variant of a
protocol (Section 2.2).
The Service Code mechanism allows an application to declare the set
of services that are associated with server port numbers. This can
affect how an application interacts with DCCP. It also allows
decoupling of the role of port numbers to indicate a desired service
from the role of port numbers in demultiplexing and state management.
A DCCP application identifies the requested service by the Service
Code value in a DCCP-Request packet. Each application therefore
associates one or more Service Codes with each listening port
([RFC4340], Section 8.1.2).
The use of Service Codes can assist in identifying the intended
service by a firewall and may assist other middleboxes (e.g., a proxy
server or network address translator (NAT) [RFC2663]). Middleboxes
that desire to identify the type of data a flow claims to transport
should utilize the Service Code for this purpose. When consistently
used, the Service Code can provide a more specific indication of the
actual service (e.g., indicating the type of multimedia flow or
intended application behaviour) than deriving this information from
the server port value.
The more flexible use of server ports can also offer benefits to
applications where servers need to handle very large numbers of
simultaneous-open ports to the same service.
RFC 4340 omits a description of the motivation behind Service Codes,
and it does not properly describe how Well Known and Registered
server ports relate to Service Codes. The intent of this document is
to clarify these issues.
RFC 4340 states that Service Codes are not intended to be DCCP-
specific. Service Codes, or similar concepts, may therefore also be
useful to other IETF transport protocols.
1.1. History
It is simplest to understand the motivation for defining Service
Codes by describing the history of the DCCP protocol.
Fairhurst Standards Track [Page 3]
RFC 5595 DCCP Service Codes September 2009
Most current Internet transport protocols (TCP [RFC793], UDP
[RFC768], SCTP (the Stream Control Transmission Protocol) [RFC4960],
and UDP-Lite [RFC3828]) use "Published" port numbers from the Well
Known or Registered number spaces [RFC814]. These 16-bit values
indicate the application service associated with a connection or
message. The server port must be known to the client to allow a
connection to be established. This may be achieved using out-of-band
signalling (e.g., described using SDP [RFC4566]), but more commonly a
Published port is allocated to a particular protocol or application;
for example, HTTP commonly uses port 80 and SMTP commonly uses port
25. Making a port number Published [RFC1122] involves registration
with the Internet Assigned Numbers Authority (IANA), which includes
defining a service by a unique keyword and reserving a port number
from among a fixed pool [IANA].
In the earliest draft of DCCP, the authors wanted to address the
issue of Published ports in a future-proof manner, since this method
suffers from several problems:
o The port space is not sufficiently large for ports to be easily
allocated (e.g., in an unregulated manner). Thus, many
applications operate using unregistered ports, possibly colliding
with use by other applications.
o The use of port-based firewalls encourages application writers to
disguise one application as another in an attempt to bypass
firewall filter rules. This motivates firewall writers to use
deep packet inspection in an attempt to identify the service
associated with a port number.
o ISPs often deploy transparent proxies, primarily to improve
performance and reduce costs. For example, TCP requests destined
to TCP port 80 are often redirected to a web proxy.
These issues are coupled. When applications collide on the same
Published-but-unregistered port, there is no simple way for network
security equipment to tell them apart, and thus it is likely that
problems will be introduced through the interaction of features.
There is little that a transport protocol designer can do about
applications that attempt to masquerade as other applications. For
ones that are not attempting to hide, the problem may be simply that
they cannot trivially obtain a Published port. Ideally, it should be
sufficiently easy that every application writer can request a Well
Known or Registered port and receive one instantly with no questions
asked. The 16-bit port space traditionally used is not large enough
to support such a trivial allocation of ports.
Fairhurst Standards Track [Page 4]
RFC 5595 DCCP Service Codes September 2009
Thus, the designers of DCCP sought an alternative solution. The idea
was simple. A 32-bit server port space should be sufficiently large
to enable use of very simple allocation policies. However, overhead
considerations made a 32-bit port value undesirable (DCCP needed to
be useful for low-rate applications).
The solution in DCCP to this problem was to use a 32-bit Service Code
[RFC4340] that is included only in the DCCP-Request packet. The use
of a 32-bit value was intended to make it trivially simple to obtain
a unique value for each application. Placing the value in a DCCP-
Request packet requires no additional overhead for the actual data
flow. It is however sufficient for both the end systems, and
provides any stateful middleboxes along the path with additional
information to understand what applications are being used.
Early discussion of the DCCP protocol considered an alternative to
the use of traditional ports; instead, it was suggested that a client
use a 32-bit identifier to uniquely identify each connection and that
the server listen on a socket bound only to a Service Code. This
solution was unambiguous; the Service Code was the only identifier
for a listening socket at the server side. The DCCP client included
a Service Code in the request, allowing it to reach the corresponding
listening application. One downside was that this prevented
deployment of two servers for the same service on a single machine,
something that is trivial with ports. The design also suffered from
the downside of being sufficiently different from existing protocols
that there were concerns that it would hinder the use of DCCP through
NATs and other middleboxes.
RFC 4340 abandoned the use of a 32-bit connection identifier in favor
of two traditional 16-bit port values, one chosen by the server and
one by the client. This allows middleboxes to utilize similar
techniques for DCCP, UDP, TCP, etc. However, it introduced a new
problem: "How does the server port relate to the Service Code?" The
intent was that the Service Code identified the application or
protocol using DCCP, providing middleboxes with information about the
intended use of a connection, and that the pair of ports effectively
formed a 32-bit connection identifier, which was unique between a
pair of end systems.
The large number of available, unique Service Code values allows all
applications to be assigned a unique Service Code. However, there
remained a problem: the server port was chosen by the server, but the
client needed to know this port to establish a connection. It was
undesirable to mandate out-of-band communication to discover the
server port. The chosen solution was to register DCCP server ports.
The limited availability of DCCP server ports appears to contradict
the benefits of DCCP Service Codes because, although it may be
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RFC 5595 DCCP Service Codes September 2009
trivial to obtain a Service Code, it has not traditionally been
trivial to obtain a Registered port from IANA and, in the long-run,
it may not be possible to allocate a unique Registered DCCP port to
new applications. As port numbers become scarce, this motivates the
need to associate more than one Service Code with a listening port
(e.g., two different applications could be assigned the same server
port and need to run on the same host at the same time,
differentiated by their different associated Service Codes).
Service Codes provide flexibility in the way clients identify the
server application to which they wish to communicate. The mechanism
allows a server to associate a set of server ports with a service.
The set may be common with other services available at the same
server host, allowing a larger number of concurrent connections for a
particular service than possible when the service is identified by a
single Published port number.
1.2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. An Architecture for Service Codes
DCCP defines the use of a combination of ports and Service Codes to
identify the server application ([RFC4340], Section 8.1.2). These
are described in the following sections.
2.1. IANA Port Numbers
In DCCP, the packets belonging to a connection are demultiplexed
based on a combination of four values {source IP address, source
port, dest IP address, dest port}, as in TCP. An endpoint address is
associated with a port number (e.g., forming a socket) and a pair of
associations uniquely identifies each connection. Ports provide the
fundamental per-packet demultiplexing function.
The Internet Assigned Numbers Authority currently manages the set of
globally reserved port numbers [IANA]. The source port associated
with a connection request, often known as the "ephemeral port", is
traditionally in the range 49152-65535 and also includes the range
1024-49151. The value used for the ephemeral port is usually chosen
by the client operating system. It has been suggested that a
randomized choice port number value can help defend against "blind"
attacks [Rand] in TCP. This method may be applicable to other IETF-
defined transport protocols, including DCCP.
Fairhurst Standards Track [Page 6]
RFC 5595 DCCP Service Codes September 2009
Traditionally, the destination (server) port value associated with a
service is determined either by an operating system index that points
to a copy of the IANA table (e.g., getportbyname() in Unix, which
indexes the /etc/services file) or by the application specifying a
direct mapping.
The UDP and TCP port number space: 0..65535, is split into three
ranges [RFC2780]:
o 0..1023 "Well Known", also called "system" ports,
o 1024..49151 "Registered", also called "user" ports, and
o 49152..65535 "Dynamic", also called "private" ports.
DCCP supports Well Known and Registered ports. These are allocated
in the DCCP IANA Port Numbers registry ([RFC4340], Section 19.9).
Each Registered DCCP port MUST be associated with at least one pre-
defined Service Code.
Applications that do not need to use a server port in the Well Known
or Registered range SHOULD use a Dynamic server port (i.e., one not
required to be registered in the DCCP Port registry). Clients can
identify the server port value for the services to which they wish to
connect using a range of methods. One common method is by reception
of an SDP record (Section 2.6) exchanged out-of-band (e.g., using SIP
[RFC3261] or the Real Time Streaming Protocol (RTSP) [RFC2326]). DNS
SRV resource records also provide a way to identify a server port for
a particular service based on the service's string name [RFC2782].
Applications that do not use out-of-band signalling can still
communicate, provided that both client and server agree on the port
value to be used. This eliminates the need for each registered
Service Code to be allocated to an IANA-assigned server port (see
also Section 2.7).
2.2. DCCP Service Code Values
DCCP specifies a 4-byte Service Code ([RFC4340], Section 8.1.2)
represented in one of three forms: a decimal number (the canonical
method), a 4-character ASCII string [ANSI.X3-4.1986], or an 8-digit
hexadecimal number. All standards assigned Service Codes, including
all values assigned by IANA, are required to use a value that may be
represented using a subset of the ASCII character set. Private
Service Codes do not need to follow this convention, although RFC
4340 suggests that users choose Service Codes that may also be
represented in ASCII.
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RFC 5595 DCCP Service Codes September 2009
The Service Code identifies the application-level service to which a
client application wishes to connect. For example, services have
been defined for the Real-Time Protocol (RTP) [RTP-DCCP]. In a
different example, Datagram Transport Layer Security (DTLS) [RFC5238]
provides a transport-service (not an application-layer service);
therefore, applications using DTLS are individually identified by a
set of corresponding Service Code values.
Endpoints MUST associate a Service Code with every DCCP socket
[RFC4340], both actively and passively opened. The application will
generally supply this Service Code. A single passive-listening port
may be associated with more than one Service Code value. The set of
Service Codes could be associated with one or more server
applications. This permits a more flexible correspondence between
services and port numbers than is possible using the corresponding
socket pair (4-tuple of layer-3 addresses and layer-4 ports). In the
currently defined set of packet types, the Service Code value is
present only in DCCP-Request ([RFC4340], Section 5.2) and DCCP-
Response packets ([RFC4340], Section 5.3). Note that new DCCP packet
types (e.g., [RFC5596]) could also carry a Service Code value.
2.2.1. New Versions of Applications or Protocols
Applications/protocols that provide version negotiation or indication
in the protocol operating over DCCP do not require a new server port
or new Service Code for each new protocol version. New versions of
such applications/protocols SHOULD continue to use the same Service
Code. If the application developers feel that the new version
provides significant new capabilities (e.g., that will change the
behavior of middleboxes), they MAY allocate a new Service Code
associated with the same or different set of Well Known ports. If
the new Service Code is associated with a Well Known or Registered
port, the DCCP Ports registry MUST also be updated to include the new
Service Code value, but MAY share the same server port assignment(s).
2.3. Service Code Registry
The set of registered Service Codes specified for use within the
general Internet are defined in an IANA-controlled name space. IANA
manages new allocations of Service Codes in this space [RFC4340].
Private Service Codes are not centrally allocated and are denoted by
the decimal range 1056964608-1073741823 (i.e., 32-bit values with the
high-order byte equal to a value of 63, corresponding to the ASCII
character '?').
Associations of Service Code with Well Known ports are also defined
in the IANA DCCP Port registry (Section 2.1).
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RFC 5595 DCCP Service Codes September 2009
2.4. Zero Service Code
A Service Code of zero is "permanently reserved (it represents the
absence of a meaningful Service Code)" [RFC4340]. This indicates
that no application information was provided. RFC 4340 states that
applications MAY be associated with this Service Code in the same way
as other Service Code values. This use is permitted for any server
port.
This document clarifies Section 19.8 of RFC 4340 by adding the
following:
Applications SHOULD NOT use a Service Code of zero.
Application writers that need a temporary Service Code value
SHOULD choose a value from the private range (Section 2.3).
Applications intended for deployment in the Internet are
encouraged to use an IANA-defined Service Code. If no specific
Service Code exists, they SHOULD request a new assignment from the
IANA.
2.5. Invalid Service Code
RFC 4340 defines the Service Code value of 4294967295 in decimal
(0xFFFFFFFF) as "invalid". This is provided so implementations can
use a special 4-byte value to indicate "no valid Service Code".
Implementations MUST NOT accept a DCCP-Request with this value, and
SHOULD NOT allow applications to bind to this Service Code value
[RFC4340].
2.6. SDP for Describing Service Codes
Methods that currently signal destination port numbers, such as the
Session Description Protocol (SDP) [RFC4566], require an extension to
support DCCP Service Codes [RTP-DCCP].
2.7. A Method to Hash the Service Code to a Dynamic Port
Applications that do not use out-of-band signalling or an IANA-
assigned port still require both the client and server to agree on
the server port value to be used. This section describes an optional
method that allows an application to derive a default server port
number from the Service Code. The returned value is in the Dynamic
port range [RFC4340]:
Fairhurst Standards Track [Page 9]
RFC 5595 DCCP Service Codes September 2009
int s_port; /* server port */
s_port = ((sc[0]<<7)^(sc[1]<<5)^(sc[2]<<3)^sc[3]) | 0xC000;
if (s_port==0xFFFF) {s_port = 0xC000;}
where sc[] represents the 4 bytes of the Service Code, and sc[3] is
the least significant byte. For example, this function associates
SC:fdpz with the server port 64634.
This algorithm has the following properties:
o It identifies a default server port for each service.
o It seeks to assign different Service Codes to different ports, but
does not guarantee an assignment is unique.
o It preserves the 4 lowest bits of the final bytes of the Service
Code, which allows many common series of Service Codes to be
mapped to a set of adjacent port numbers, e.g., Foo1, and Foo2;
Fooa and Foob would be assigned adjacent ports. (Note: this
consecutive numbering only applies to characters in the range 0-9
and A-O and P-Z. When the characters cross a range boundary, the
algorithm introduces a discontinuity, resulting in mapping to
non-consecutive ports. Hence, Fooo and Foop respectively map to
the decimal values of 65015 and 65000).
o It avoids the port 0xFFFF, which is not accessible on all host
platforms.
Applications and higher-layer protocols that have been assigned a
Service Code (or use a Service Code from the unassigned private
space) may use this method. It does not preclude other applications
using the selected server port, since DCCP servers are differentiated
by the Service Code value.
3. Use of the DCCP Service Code
The basic operation of Service Codes is as follows:
A client initiating a connection:
- issues a DCCP-Request with a Service Code and chooses a
destination (server) port number that is expected to be
associated with the specified Service Code at the destination.
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RFC 5595 DCCP Service Codes September 2009
A server that receives a DCCP-Request:
- determines whether an available service matching the Service
Code is supported for the specified destination server port.
The session is associated with the Service Code and a
corresponding server. A DCCP-Response is returned.
- if the service is not available, the session is rejected and a
DCCP-Reset packet is returned.
3.1. Setting Service Codes at the Client
A client application MUST associate every DCCP connection (and hence
every DCCP active socket) with a single Service Code value
[RFC4340]). This value is used in the corresponding DCCP-Request
packet.
3.2. Using Service Codes in the Network
DCCP connections identified by the Service Code continue to use IP
addresses and ports, although neither port number may be Published.
Port numbers and IP addresses are the traditional methods to identify
a flow within an IP network. Middlebox [RFC3234] implementors
therefore need to note that new DCCP connections are identified by
the pair of server port and Service Code in addition to the IP
address. This means that the IANA may allocate a server port to more
than one DCCP application [RFC4340].
Network address and port translators, known collectively as NATs
[RFC2663], may interpret DCCP ports ([RFC2993] and [RFC5597]). They
may also interpret DCCP Service Codes. Interpreting DCCP Service
Codes can reduce the need to correctly interpret port numbers,
leading to new opportunities for network address and port
translators. Although it is encouraged to associate specific
delivery properties with the Service Code, e.g., to identify the
real-time nature of a flow that claims to be using RTP, there is no
guarantee that the actual connection data corresponds to the
associated Service Code. A middlebox implementor may still use deep
packet inspection, and other means, in an attempt to verify the
content of a connection.
The use of the DCCP Service Code can potentially lead to interactions
with other protocols that interpret or modify DCCP port numbers
[RFC3234]. The following additional clarifications update the
description provided in Section 16 of RFC 4340:
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RFC 5595 DCCP Service Codes September 2009
o A middlebox that intends to differentiate applications SHOULD
test the Service Code in addition to the destination or source
port of a DCCP-Request or DCCP-Response packet.
o A middlebox that does not modify the intended application
(e.g., NATs [RFC5597] and Firewalls) MUST NOT change the
Service Code.
o A middlebox MAY send a DCCP-Reset in response to a packet with
a Service Code that is considered unsuitable.
3.3. Using Service Codes at the Server
The combination of the Service Code and server port disambiguates
incoming DCCP-Requests received by a server. The Service Code is
used to associate a new DCCP connection with the corresponding
application service. Four cases can arise when two DCCP server
applications passively listen on the same host:
o The simplest case arises when two servers are associated with
different Service Codes and are bound to different server ports
(Section 3.3.1).
o Two servers may be associated with the same DCCP Service Code
value but be bound to different server ports (Section 3.3.2).
o Two servers could use different DCCP Service Code values and be
bound to the same server port (Section 3.3.1).
o Two servers could attempt to use the same DCCP Service Code and
bind to the same server port. A DCCP implementation MUST
disallow this, since there is no way for the DCCP host to
direct a new connection to the correct server application.
RFC 4340 (Section 8.1.2) states that an implementation:
o MUST associate each active socket with exactly one Service Code
on a specified server port.
In addition, Section 8.1.2 of RFC 4340 also states:
o Passive sockets MAY, at the implementation's discretion, be
associated with more than one Service Code; this might let
multiple applications, or multiple versions of the same
application, listen on the same port, differentiated by Service
Code.
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RFC 5595 DCCP Service Codes September 2009
This document updates the above text from RFC 4340 by replacing it
with the following:
o An implementation SHOULD allow more than one Service Code to be
associated with a passive server port, enabling multiple
applications, or multiple versions of an application, to listen
on the same port, differentiated by the associated Service
Code.
It also adds:
o An implementation SHOULD provide a method that informs a server
of the Service Code value that was selected by an active
connection.
A single passively opened (listening) port MAY therefore be
associated with multiple Service Codes, although an active (open)
connection can only be associated with a single Service Code. A
single application may wish to accept connections for more than one
Service Code using the same server port. This may allow a server to
offer more than the limit of 65,536 services depending on the size of
the Port field. The upper limit is based solely on the number of
unique connections between two hosts (i.e., 4,294,967,296).
3.3.1. Reception of a DCCP-Request
When a DCCP-Request is received and the specified destination port is
not bound to a server, the host MUST reject the connection by issuing
a DCCP-Reset with the Reset Code "Connection Refused". A host MAY
also use the Reset Code "Too Busy" ([RFC4340], Section 8.1.3).
When the requested destination port is bound to a server, the host
MUST also verify that the server port is associated with the
specified Service Code (there could be multiple Service Code values
associated with the same server port). Two cases can occur:
o If the receiving host is listening on a server port and the DCCP-
Request uses a Service Code that is associated with the port, the
host accepts the connection. Once connected, the server returns a
copy of the Service Code in the DCCP-Response packet, completing
the initial handshake [RFC4340].
o If the server port is not associated with the requested Service
Code, the server SHOULD reject the request by sending a DCCP-Reset
packet with the Reset Code 8, "Bad Service Code" ([RFC4340],
Section 8.1.2), but MAY use the reason "Connection Refused".
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RFC 5595 DCCP Service Codes September 2009
After a connection has been accepted, the protocol control block is
associated with a pair of ports, a pair of IP addresses, and a single
Service Code value.
3.3.2. Multiple Associations of a Service Code with Ports
DCCP Service Codes are not restricted to specific ports, although
they may be associated with a specific Well Known port. This allows
the same DCCP Service Code value to be associated with more than one
server port (in either the active or passive state).
3.3.3. Automatically Launching a Server
A host implementation may permit a service to be associated with a
server port (or range of ports) that is not permanently running at
the server. In this case, the arrival of a DCCP-Request may require
a method to associate a DCCP-Request with a server that handles the
corresponding Service Code. This operation could resemble that of
"inetd" [inetd].
As in the previous section, when the specified Service Code is not
associated with the specified server port, the connection MUST be
aborted and a DCCP Reset message sent [RFC4340].
4. Security Considerations
The security considerations of RFC 4340 identify and offer guidance
on security issues relating to DCCP. This document discusses the
usage of Service Codes. It does not describe new protocol functions.
All IPsec modes protect the integrity of the DCCP header. This
protects the Service Code field from undetected modification within
the network. In addition, the IPsec Encapsulated Security Payload
(ESP) mode may be used to encrypt the Service Code field, hiding the
Service Code value within the network and also preventing
interpretation by middleboxes. The DCCP header is not protected by
application-layer security (e.g., the use of DTLS [RFC5238] as
specified in DTLS/DCCP [RFC4347]).
There are four areas of security that are important:
1. Server Port number reuse (Section 4.1).
2. Interaction with NATs and firewalls (Section 3.2 describes
middlebox behavior). Requirements relating to DCCP are described
in [RFC5597].
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3. Interpretation of DCCP Service Codes overriding traditional use of
reserved/Well Known port numbers (Section 4.2).
4. Interaction with IPsec and DTLS security (Section 4.3).
4.1. Server Port Number Reuse
Service Codes are used in addition to ports when demultiplexing
incoming connections. This changes the service model to be used by
applications and middleboxes. The Port Numbers registry already
contains instances of multiple application registrations for a single
port number for TCP and UDP. These are relatively rare. Since the
DCCP Service Code allows multiple applications to safely share the
same port number, even on the same host, server port number reuse in
DCCP may be more common than in TCP and UDP.
4.2. Association of Applications with Service Codes
The use of Service Codes provides more ready feedback that a concrete
service is associated with a given port on a server than for a
service that does not employ Service Codes. By responding to an
inbound connection request, systems not using these codes may
indicate that some service is, or is not, available on a given port,
but systems using this mechanism immediately provide confirmation (or
denial) that a particular service is present. This may have
implications in terms of port scanning and reconnaissance.
Care needs to be exercised when interpreting the mapping of a Service
Code value to the corresponding service. The same service
(application) may be accessed using more than one Service Code.
Examples include the use of separate Service Codes for an application
layered directly upon DCCP and one using DTLS transport over DCCP
[RFC5238]. Other possibilities include the use of a private Service
Code to map to an application that has already been assigned an IANA-
defined Service Code value, or multiple Service Code values that map
to a single application providing more than one service. Different
versions of a service (application) may also be mapped to a
corresponding set of Service Code values.
Processing of Service Codes may imply more processing than currently
associated with incoming port numbers. Implementors need to guard
against increasing opportunities for Denial of Service attacks.
4.3. Interactions with IPsec
The Internet Key Exchange protocol (IKEv2) does not currently specify
a method to use DCCP Service Codes as a part of the information used
to set up an IPsec security association.
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IPsec uses port numbers to perform access control in transport mode
[RFC4301]. Security policies can define port-specific access control
(PROTECT, BYPASS, DISCARD) as well as port-specific algorithms and
keys. Similarly, firewall policies allow or block traffic based on
port numbers.
Use of port numbers in IPsec selectors and firewalls may assume that
the numbers correspond to Well Known services. It is useful to note
that there is no such requirement; any service may run on any port,
subject to mutual agreement between the endpoint hosts. Use of the
Service Code may interfere with this assumption both within IPsec and
within other firewall systems, but it does not add a new
vulnerability. New implementations of IPsec and firewall systems may
interpret the Service Code when implementing policy rules, but should
not rely on either port numbers or Service Codes to indicate a
specific service.
5. IANA Considerations
This document does not update the IANA allocation procedures for the
DCCP Port Number and DCCP Service Codes Registries as defined in RFC
4340.
For completeness, the document notes that it is not required to
supply an approved document (e.g., a published RFC) to support an
application for a DCCP Service Code or port number value, although
RFCs may be used to request Service Code values via the IANA
Considerations section. A specification is however required to
allocate a Service Code that uses a combination of ASCII digits,
uppercase letters, and character space, '-', '.', and '/') [RFC4340].
6. Acknowledgments
This work has been supported by the EC IST SatSix Project.
Significant contributions to this document resulted from discussion
with Joe Touch, and this is gratefully acknowledged. The author also
thanks Ian McDonald, Fernando Gont, Eddie Kohler, and the DCCP WG for
helpful comments on this topic, and Gerrit Renker for his help in
determining DCCP behavior and review of this document. Mark Handley
provided significant input to the text on the definition of Service
Codes and their usage. He also contributed much of the material that
has formed the historical background section.
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7. References
7.1. Normative References
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC5597] Denis-Courmont, R., "Network Address Translation (NAT)
Behavioral Requirements for the Datagram Congestion
Control Protocol", BCP 150, RFC 5597, September 2009.
7.2. Informative References
[ANSI.X3-4.1986]
American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[IANA] Internet Assigned Numbers Authority, www.iana.org.
[RTP-DCCP] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", Work in Progress, June 2007.
[Rand] Larsen, M. and F. Gont, "Port Randomization", Work in
Progress, March 2009.
[inetd] The extended inetd project, http://xinetd.org.
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC814] Clark, D., "Name, addresses, ports, and routes", RFC 814,
July 1982.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
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[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP
37, RFC 2780, March 2000.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[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.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, July 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)", RFC
5238, May 2008.
[RFC5596] Fairhurst, G., "Datagram Congestion Control Protocol
(DCCP) Simultaneous-Open Technique to Facilitate
NAT/Middlebox Traversal", RFC 5596, September 2009.
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Author's Address
Godred Fairhurst,
School of Engineering,
University of Aberdeen,
Kings College,
Aberdeen, AB24 3UE,
UK
EMail: gorry@erg.abdn.ac.uk
URL: http://www.erg.abdn.ac.uk/users/gorry
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