Internet DRAFT - draft-ietf-dccp-serv-codes
draft-ietf-dccp-serv-codes
DCCP WG G.Fairhurst
Internet-Draft University of Aberdeen
Intended status: Proposed Standard May 26, 2009
Expires: October 31, 2009
Updates: RFC 4340
The DCCP Service Code
draft-ietf-dccp-serv-codes-11.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on October 26, 2009.
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.
G. Fairhurst Expires October 26, 2009 [Page 1]
Internet-Draft DCCP Service Codes May 2009
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..................................8
2.2.1. New versions of Applications or Protocols............8
2.3. Service Code Registry.....................................9
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......10
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 re-use................................15
4.2. Association of applications with Service Codes...........15
4.3. Interactions with IPsec..................................16
5. IANA Considerations...........................................16
6. Acknowledgments...............................................16
7. References....................................................17
7.1. Normative References.....................................17
7.2. Informative References...................................17
8. Author's Addresses............................................19
8.1. Disclaimer...............................................19
8.2. Copyright Notice.........................................19
G. Fairhurst Expires October 26, 2009 [Page 2]
Internet-Draft DCCP Service Codes May 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 a standard profile, e.g. [ID.RTP]) 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 allows decoupling
the role of port numbers to indicate a desired service from the role
in connection 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, 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).
The more flexible use of server ports can also offer benefit to
applications where servers need to handle very large numbers of
simultaneous open ports to the same service.
RFC 4340 omits to describe the motivation behind Service Codes, nor
does it properly describe how Well Known and Registered server ports
relate to Service Codes. The intent of this document is to clarify
these issues.
1.1. History
It is simplest to understand the motivation for defining Service
Codes by describing the history of the DCCP protocol.
Most current Internet transport protocols (TCP [RFC793], UDP
[RFC768], SCTP [RFC4960], UDP-Lite [RFC3828]) used "Published" port
numbers from the Well Known or registered number spaces [RFC814].
G. Fairhurst Expires October 26, 2009 [Page 3]
Internet-Draft DCCP Service Codes May 2009
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 signaling (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, with the likelihood of
introducing problems with 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.
Thus, the design of DCCP sought an alternative solution. The idea
was simple. A 32-bit server port space should be sufficiently large
that it enables use of very simple allocation policies. However,
G. Fairhurst Expires October 26, 2009 [Page 4]
Internet-Draft DCCP Service Codes May 2009
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
used a 32-bit identifier to uniquely identify each connection. The
server listened 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
remains a current problem: The server port is chosen by the server,
but the client needs to know this to establish a connection. It was
undesirable to mandate out-of-band communication to discover the
server port. A solution is 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 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
G. Fairhurst Expires October 26, 2009 [Page 5]
Internet-Draft DCCP Service Codes May 2009
possible to uniquely 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.
There has been confusion concerning how server ports relate to
Service Codes. The goal of this document is to clarify this and the
issues concerning the use of Service Codes.
RFC4340 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.
1.2. 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 de-multiplexed
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 de-multiplexing function.
G. Fairhurst Expires October 26, 2009 [Page 6]
Internet-Draft DCCP Service Codes May 2009
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 of port number value can help defend against
"blind" attacks [ID.Rand] in TCP. This method may be applicable to
other IETF-defined transport protocols, including DCCP.
Traditionally, the destination (server) port value associated with a
service is determined either by an operating system index to a copy
of the IANA table (e.g., getportbyname() in Unix, which indexes the
/etc/services file), or directly mapped by the application.
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,
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. that does
not require 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 a SDP record (Section 2.6) exchanged out-of-band (e.g. using SIP
[RFC3261] or RTSP [RFC2326]). DNS SRV resource records also provide a
way to identify a server port for a particular service based on the
services string name [RFC2782].
Applications that do not use out-of-band signalling can still
communicate, providing that both client and server agree the port
value to be used. This eliminates the need for each registered
Service Code to be allocated an IANA-assigned server port (see also
Section 2.7).
G. Fairhurst Expires October 26, 2009 [Page 7]
Internet-Draft DCCP Service Codes May 2009
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 four character ASCII string [ANSI.X3-4.1986], or an eight
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 also choose Service Codes that may also
be represented in ASCII.
The Service Code identifies the application-level service to which a
client application wishes to connect. Examples of services are RTP
[ID.RTP], TIME (this document), ECHO (this document). In a different
example, 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 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 new DCCP packet types
(e.g. [ID.Simul]) 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 a 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).
G. Fairhurst Expires October 26, 2009 [Page 8]
Internet-Draft DCCP Service Codes May 2009
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).
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
RFC4340 defines the Service Code value of 0xFFFFFFFF as Invalid. This
is provided so implementations can use a special four-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 extension to
support DCCP Service Codes [ID.RTP].
G. Fairhurst Expires October 26, 2009 [Page 9]
Internet-Draft DCCP Service Codes May 2009
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 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]:
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 four 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 four bits of the final bytes of the Service Code,
allowing mapping common series of Service Codes to adjacent ports,
e.g. Foo1, and Foo2; and 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:
G. Fairhurst Expires October 26, 2009 [Page 10]
Internet-Draft DCCP Service Codes May 2009
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.
o 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] [ID.Behave-DCCP]. 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.
G. Fairhurst Expires October 26, 2009 [Page 11]
Internet-Draft DCCP Service Codes May 2009
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:
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 [ID.Behave-DCCP] 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 also states:
G. Fairhurst Expires October 26, 2009 [Page 12]
Internet-Draft DCCP Service Codes May 2009
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."
This document updates this text in RFC 4340 by replacing this 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 determined by 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 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].
G. Fairhurst Expires October 26, 2009 [Page 13]
Internet-Draft DCCP Service Codes May 2009
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 Reset Code 8, "Bad Service Code" ([RFC4340], Section
8.1.2), but MAY use the reason "Connection Refused".
After a connection has been accepted, the protocol control block is
associated with a pair of ports and 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 identifies and offers
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 DTLS [RFC5238] as
specified in DTLS/DCCP [RFC4347]).
G. Fairhurst Expires October 26, 2009 [Page 14]
Internet-Draft DCCP Service Codes May 2009
There are four areas of security that are important:
1. Server Port number reuse (section 5.1).
2. Interaction with NATs and firewalls (section 3.2 describes
middlebox behaviour). Requirements relating to DCCP are described
in [ID.Behave-DCCP].
3. Interpretation of DCCP Service Codes over-riding traditional use
of reserved/Well Known port numbers (Section 5.2).
4. Interaction with IPsec and DTLS security (section 5.3).
4.1. Server Port number re-use
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 servers, than for a
service that does not employing 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 that maps to the same application as assigned to an IANA-defined
Service Code value, or a single application that provides more than
one service. Different versions of a service (application) may also
be mapped to a corresponding set of Service Code values.
G. Fairhurst Expires October 26, 2009 [Page 15]
Internet-Draft DCCP Service Codes May 2009
Processing of Service Codes may imply more processing than currently
associated with incoming port numbers. Implementers need to guard
against increasing opportunities for Denial of Service attack.
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 setup an IPsec security association.
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
in 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
G. Fairhurst Expires October 26, 2009 [Page 16]
Internet-Draft DCCP Service Codes May 2009
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 behaviour and review of this document. Mark Handley
provided significant input to the text on definition of Service Codes
and their usage. He also contributed much of the material that has
formed the historical background Section.
7. References
7.1. Normative References
[RFC1122] Braden, R. (ed.), "Requirements for Internet Hosts:
Communication Layers, " STD 3, RFC 1122, Oct. 1989
(STANDARD).
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997 (BEST
CURRENT PRACTICE).
[RFC4340] Kohler, E., M. Handley, S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, Mar. 2006 (PROPOSED
STANDARD).
[ID.Behave-DCCP] R. Denis-Courmont, "Network Address Translation
(NAT) Behavioral Requirements for DCCP", IETF Work in
Progress, draft-ietf-behave-dccp-05.txt.
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
[IANA.SC] IANA DCCP Service Code Registry
http://www.iana.org/assignments/service-codes
[ID.Simul] G. Fairhurst, G. Renker, "DCCP Simultaneous-Open Technique
to Facilitate NAT/Middlebox Traversal", IETF Work in
Progress, draft-ietf-dccp-simul-open-08.txt.
[ID.RTP] C. Perkins, "RTP and the Datagram Congestion Control
Protocol (DCCP)", IETF Work in Progress, draft-ietf-dccp-
rtp-07.txt.
G. Fairhurst Expires October 26, 2009 [Page 17]
Internet-Draft DCCP Service Codes May 2009
[ID.Rand] M. Larsen, F. Gont, "Port Randomization", IETF Work in
Progress, draft-larsen-tsvwg-port-randomization-02.txt
[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, Sept. 1981 (STANDARD).
[RFC814] Clark, D., "NAME, ADDRESSES, PORTS, AND ROUTES", RFC 814,
July 1982 (UNKNOWN).
[RFC862] Postel, J., "Echo Protocol", STD 20, RFC 862, May 1983.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[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., and
G. Fairhurst, "The Lightweight User Datagram Protocol (UDP-
Lite)", RFC 3828, July 2004.
G. Fairhurst Expires October 26, 2009 [Page 18]
Internet-Draft DCCP Service Codes May 2009
[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.
8. Author's Addresses
Godred (Gorry) 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
8.1. Disclaimer
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
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.
8.2. Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
G. Fairhurst Expires October 26, 2009 [Page 19]
Internet-Draft DCCP Service Codes May 2009
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
G. Fairhurst Expires October 26, 2009 [Page 20]
Internet-Draft DCCP Service Codes May 2009
>>> RFC Editor please remove this Section prior to publication.
Change Log.
01 introduced:
- a replacement of the word *range* when referring to sets of dccp
ports (they are not necessarily contiguous), noted by E. Kohler.
- Addition of some Service Codes in IANA Section.
02 introduced:
- add the use of profiles with DCCP, identified by Service Code, but
not the use of protocol variants.
- further detail on implementation levels (more input would be good)
- added security consideration for traffic generators
- added ref to UDPL for completeness
- Corrected NiTs found by Gerrit Renker
+++++++++++++++++++++++++++
WG 00 (first WG version)
This introduced revisions to make it a WG document.
- Corrected language and responded to many helpful comments from
Fernando Gont and Ian McDonald.
- Added a test for which server behaviour is used.
- Added some speculative text on how to implement the SC.
- More input and discussion is requested from the WG.
- Added an informative appendix on host configuration.
- Merging of some Sections to remove repetition and clarify wording.
+++++++++++++++++++++++++++
G. Fairhurst Expires October 26, 2009 [Page 21]
Internet-Draft DCCP Service Codes May 2009
WG 01
Historical material was added.
Comments from the list have been included.
The concept of adding weak semantics to a SC=0 was removed. This was
added at the request of implementers, with the aim of offering easier
implementation on at least one target platform. It has been removed
in this document because it weakens interoperability and complicates
the Spec.
The proposal to allow several levels of support was introduced in
previous drafts following suggestions from the WG, but was removed in
this revision. The method was seen to introduce complexity, and
resulted in complex interoperability scenarios.
Removed "test" method, this was no longer required.
Draft was reorganized to improve clarity and simplify concepts.
----
WG 02
Updated following comments from Eddie Kohler.
----
WG 03
Fixed NiTs and addressed issues marked in previous version.
Added 2 para at end of port Section saying how to use Well Known
ports and that you do not need to register them.
-----
WG 04
Cleaned English (removing duplication)
Checked text that updates RFC4340 (and remove duplicates).
Updated hash algorithm for SC->s_port
Updated to IANA Section.
G. Fairhurst Expires October 26, 2009 [Page 22]
Internet-Draft DCCP Service Codes May 2009
Edits in response to feedback from Tom Phelan, et al.
-----
WG-05:
Various Sections were updated following feedback from the list, some
specific comments were:
Tom Phelan suggested clarification was needed for the usage of well-
known ports in Section 1, and various other clarifications.
Eddie Kohler suggested reworking the midbox Section.
Eddie noted the hash function included the highest numbered port,
which is not accessible on all OS.
There was also discussion about the proper server port range to be
used with this method. After previous concerns that using registered
ports could have some (unknown) side effect, use was recommended in
the dynamic range. Text was added to this Section.
Discussions at IETF-71 lead to the idea to removing the IANA guidance
on maintaining the registries to a new document that defines the
policy across the set of transport registries.
Eddie noted that port-reuse is likely to be more common with DCCP
(security considerations).
Lars noted that rate-limiting benchmarking tools may be somewhat
undesirable, and this related to services for testing.
The text recommending an update to the IANA procedures for ports and
service codes has been moved to a TSV WG draft.
-----
WG-06:
Updated the updating paragraphs to clarify the specific clauses of
RFC 4340 are changed. Comments from Eddie and Colin.
Very minor editorial corrections.
-----
WG-07:
G. Fairhurst Expires October 26, 2009 [Page 23]
Internet-Draft DCCP Service Codes May 2009
Portname for Perf in registry changed to all lower case.
Replaced para 2 of intro and updated later parts of the introduction
(feedback in LC from Eddie).
Added citation to the Behave WG Requirements for NATs (now in LC).
-----
WG-08:
New text to address editorial corrections proposed by Alfred Hoenes.
-----
WG-09:Update following review feedback
Gen-ART
Section 3.2: Middlebox [RFC3234] implementors therefore need to note
that new DCCP connections are identified by the pair of Server Port
and Service Code. - Added "in addition to the IP address" to the end
of the above sentence for clarity.
Section 3.2: Updated sentence to read: This means that the IANA may
allocate a server port to more than one DCCP application [RFC4340].
Section 3.3.2 rewritten as: DCCP Service Codes are not restricted to
specific ports, although they may be associated with a specific well-
known port. The same DCCP Service Code value may therefore be
associated with more than one server port (in either the active or
passive state).
Section 5.3: Added: 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 setup an IPsec security association.
Sec-Dir
Section 5: Added: The security considerations of RFC 4340 identifies
and offers guidance on security issues relating to DCCP.
Section 5.2: Added new paragraph: The use of Service Codes provides
more ready feedback that a concrete service is associated with a
given port on a servers, than for a service that does not employing
service codes. By responding to an inbound connection request,
systems not using these codes may indicate that some service is, or
G. Fairhurst Expires October 26, 2009 [Page 24]
Internet-Draft DCCP Service Codes May 2009
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.
-----
WG-10:Update following IESG review feedback
Typo reported by Iain Calder was fixed: simply to obtain
s/simply/simple/.
Fixed syntax error reported by Jari in the sample pseudo code, and
added more discussion of the algorithm.
A clarification of ASCII usage, suggested by:
Added text: /a four character ASCII string [ANSI.X3-4.1986], or an
eight digit hexadecimal number. All standards assigned values,
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 also choose Service Codes that may also
be represented in ASCII./
Added new informational reference:
American National Standards Institute, "Coded
Character Set - 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.
URL to iperf changed, since we note CAIDA intends to shutdown all
services associated with the NLANR.NET domain in May 2009.
section 3.3 changed to correct section references (error noted by
Ralph Droms) and additional text added to clarify sections 3.3.1 and
3.3.2. New text includes:
/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:/
G. Fairhurst Expires October 26, 2009 [Page 25]
Internet-Draft DCCP Service Codes May 2009
WG-11: Update following discussion with AD
After discussion, the section on benchmarking was removed, and will
be addressed separately.
Note: This I-D will be a normative reference in draft-ietf-dccp-
simul-open.
G. Fairhurst Expires October 26, 2009 [Page 26]
