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Versions: (draft-fairhurst-dccp-serv-codes) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5595

DCCP WG                                                     G.Fairhurst
Internet Draft                                   University of Aberdeen
Expires: September 2007                                 October 2, 2007

                           The DCCP Service Code

Status of this Memo

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   This Internet-Draft will expire on April 2, .


   This document describes the usage of Service Codes by the Datagram
   Congestion Control Protocol, RFC 4340. This document motivates the
   setting of Service Codes 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). It updates the description provided in RFC 4340.

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Table of Contents

   1. Introduction...................................................3
      1.1. History...................................................3
      1.2. Conventions used in this document.........................6
   2. An Architecture for Service Codes..............................7
      2.1. IANA Port Numbers.........................................7
      2.2. DCCP Service Code Values..................................8
      2.3. Service Code Registry.....................................8
      2.4. Zero Service Code.........................................9
      2.5. SDP for describing Service Codes..........................9
   3. Use of the DCCP Service Code...................................9
      3.1. Setting Service Codes at the Sender......................10
      3.2. Using Service Codes in the Network.......................10
      3.3. Using Service Codes at the Receiver......................11
         3.3.1. Reception of a DCCP-Request.........................12
         3.3.2. Multiple Associations of Service Codes and Ports....12
         3.3.3. Automatically launching a Server....................13
   4. Benchmarking Services Described in this document..............13
      4.1. Echo.....................................................13
      4.2. Daytime..................................................13
      4.3. Character generator......................................14
      4.4. Time service.............................................14
      4.5. PerfTest service.........................................14
   5. Security Considerations.......................................15
      5.1. Interactions of Service Codes and port numbers...........15
      5.2. Interactions with IPsec..................................16
   6. IANA Considerations...........................................16
      6.1. Port number values allocated by this document............17
      6.2. Service Code values allocated by this document...........17
   7. Acknowledgments...............................................18
   8. References....................................................19
      8.1. Normative References.....................................19
      8.2. Informative References...................................19
   9. Author's Addresses............................................21
      9.1. Intellectual Property Statement..........................21
      9.2. Disclaimer of Validity...................................21
      9.3. Copyright Statement......................................22
   APPENDIX A: API support for Service Codes........................23

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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.DCCP.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

   Service Codes allow a flexible correspondence between application-
   layer services and port numbers, which affects how applications
   interact with DCCP. This decouples the use of ports for connection
   demultiplexing and state management from their use to indicate a
   desired service. Only one application may listen on a specific port
   at any time, however when accepting a new connection, a port may be
   associated with more than one Service Code (the requested Service
   Code may then select the application).

   The use of Service Codes can assist in identifying the intended
   service when the server by a Middleboxes (a network address
   translator (NAT) [RFC2663], NAT-PT [RFC2766], Firewalls, etc).
   Middleboxes that desire to identify the type of data being
   transported by a flow, 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).

   Use of a Service Code value, instead of binding a service to a
   particular publicly-known port number, permits a larger number of
   concurrent connections for a particular service. For example, this
   may be useful for 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 describe properly how well-known 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. In the earliest
   draft of DCCP the authors wanted to address the issue of well-known
   ports in a future-proof manner.

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   Most current Internet transport protocols used "well-known" port
   numbers [RFC814]. These 16-bit values indicate the application
   service associated with a connection or message; this includes TCP
   [RFC793], UDP [RFC768], SCTP [RFC2960], UDP-Lite [RFC3828], and DCCP
   [RFC4340]. The server port must be known to the client to allow a
   connection to be established.  This could be achieved using out-of-
   band signaling, but more commonly a well-known 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 well-
   known 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].

   This method of using well-known ports 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 on 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 encourages firewall writers to use
      deep packet inspection in an attempt to validate that the
      application actually is that 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
   "well-known", but unregistered port, there is no simple way for
   network security equipment to tell them apart, with the likelihood of
   introducing feature interaction problems.

   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 well-known port.  Ideally, it should
   be sufficiently easy that every application-writer can request a
   well-known port and get one instantly with no questions asked. The
   16-bit port space traditionally used is not large enough to support
   such a trivial allocation of well-known 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,

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   overhead considerations made a 32-bit port value undesirable (DCCP
   needed to be useful for low bitrate applications).

   The solution in DCCP to this problem was the use of a 32-bit Service
   Code [RFC4340] that is included only in the DCCP-Request packet. This
   was intended to perform the primary role of a well-known server port,
   in that it would be trivially simply to obtain a unique value for
   each application. Placing the value in a 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.

   The original draft of the DCCP specification did not use traditional
   ports at all; instead the client allocated a 32-bit connection
   identifier that uniquely identified the 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, and the DCCP client would have had to
   include a Service Code in the request to allow it to reach the
   listening application.  This design 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

   RFC 43404 abandoned the use of a 32-bit connection identifier in
   favour of two traditional 16-bit ports, one chosen by the server and
   one by the client. This allows middleboxes to utilize similar
   techniques for DCCP, UDP, TCP, etc. (e.g. NAT).  This also has the
   advantage that two servers associated with the same Service Code
   could co-exist on the same server host.  However it introduced a new
   problem; "How does the server port relate to the Service Code?"  The
   intent was that the Service Code maintained its original role of
   being the globally-unique identifier for the application or protocol
   being used over DCCP, and that the pair of ports would effectively be
   a 32-bit connection identifier, unique at both end-systems, even
   though the two parts were allocated by the two different ends of a

   The large number of available unique Service Code values allows all
   applications to be assigned a 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.

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   The solution is to register well-known DCCP ports.  The limited
   availability of well-known 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 well-
   known port from IANA and in the long-run it may not be possible to
   uniquely allocate a well-known 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 well-known port, and need to
   run on the same host at the same time). The co-existence that arise
   when one port is associated with two Service Codes that are each
   bound to different applications does not raise any protocol issues.
   An incoming DCCP-Request is directed to the correct application.

   This has led to confusion concerning how well-known ports relate to
   well-known service codes. The goal of this document is to clarify the
   issues concerning the use and allocation of Service Codes.

   Service Codes provide flexibility in the way clients identify the
   server application to which they wish to communicate. Traditionally
   IANA has allocated a single well-known port value for global use by
   all hosts [RFC1122] on the public Internet, even though the
   association between a port and a service is of interest only to the
   hosts participating in a connection. This has resulted in the fixed
   space of port numbers being globally reserved unnecessarily
   [ID.Portnames]. Service Codes offer a flexible alternative where this
   name space is allocated per-host.  This allows clients to choose
   which destination port is used for a service, permitting servers to
   associate more than one port with a service and enabling a larger
   number of concurrent connections for a particular service than
   possible using well-known port numbers.

   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   All protocol code points and values are transmitted in network byte
   order (most significant byte first), with the most significant bit of
   each byte is placed in the left-most position of an 8-bit field.

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2. An Architecture for Service Codes

   DCCP defines the use of a combination of ports and Service Codes to
   identify the server application ([RFC4340], 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, forming a socket; and a pair of
   sockets uniquely identifies each connection. Ports provide the
   fundamental per-packet de-multiplexing 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",
   traditionally includes the range 49152-65535, and should also include
   the 1024-49151 range.  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.TSVWG.RAND] in TCP. Such methods may be
   applicable to other IETF-defined transport protocols, including DCCP.

   Traditionally, the destination port value that is 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 reserved ports, which are allocated in
   the DCCP IANA port registry [RFC4340].

   This section updates section 19.9 of [RFC4340] in the following way:

   "Each DCCP port entered in this registry MUST be associated with at
   least one pre-defined Service Code (see section 2.2)."

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2.2. DCCP Service Code Values

   DCCP specifies a 4 byte Service Code [RFC4340], represented in one of
   three forms as: a decimal number (the canonical method), a four
   character ASCII string, or an eight digit hexadecimal number.

   The Service Code identifies the application-level service to which a
   client application wishes to connect. Examples of services are RTP,
   TIME, ECHO. In a different example, DTLS provides a transport-service
   (not an application-layer service), therefore applications using DTLS
   are individually identified by a set of corresponding service codes.

   A single destination port may be associated with more than one
   Service Code value. These may be associated with one or different
   server applications. Appendix A provides some examples of ways that
   may be used to configure this support.

   Endpoints MUST associate a Service Code with every DCCP socket, both
   actively and passively opened. The application will generally supply
   this Service Code. This Service Code value is present only in DCCP-
   Request and DCCP-Response packets. It 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). This decouples the use of ports for connection
   demultiplexing and state management, from their use to indicate a
   desired endpoint service.

   Applications/protocols that provide version negotiation or indication
   in the protocol operating over DCCP do not require a new port for a
   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 (which MAY be associated with
   the same set of DCCP well-known ports).

2.3. Service Code Registry

   The set of registered Service Codes currently 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 range 1056964608-1073741823 (i.e. whose first
   hexadecimal digit has the ASCII value for '?').

   Associations of Service Code with Well-Known Ports may also be
   defined in the IANA DCCP Port Registry.

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2.4. Zero Service Code

   RFC 4340 assigns a Service Code of zero to represent the absence of a
   meaningful Service Code. Applications SHOULD NOT associate themselves
   with this Service Code. Application writers that need a new Service
   Code value should either choose a value from the private range
   (Section 2.3), or request a new service code from the IANA.

   >>> Author Note: The following alternative has also been proposed.

   A DCCP Service Code with a value of zero has no special meaning and
   should not be used as a wildcard service code. This section updates
   Section 19.8 of RFC 4340:

   "Implementations MUST allow applications to listen with a Service
   Code of zero, by being explicity associating a port with this Service

   >>> End Author Note

2.5. SDP for describing Service Codes

   Methods that currently signal destination port numbers, such as the
   Session Description Protocol (SDP) require extension to also support
   DCCP Service Codes [ID.DCCP.RTP].

3. Use of the DCCP Service Code

   The basic operation of the Service Codes is as follows:

   o  A sending host:

       .  issues a DCCP-Request with a Service Code and choose a
          destination 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 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.

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   This section explicitly updates RFC 4340 as follows:

   "A DCCP implementation MUST allow multiple applications using
   different DCCP service codes to listen on the same server port.

   A DCCP implementation SHOULD provide a method that informs a server
   of the Service Code value that was selected by an active connection."

   The remainder of this section describes processing of DCCP Service
   Codes at the sending and receiving hosts and within the network by

3.1. Setting Service Codes at the Sender

   A client application MUST associate every DCCP connection (and hence
   every DCCP active socket) with a single Service Code value. Valid
   Service Codes may be selected from the set of values assigned in the
   DCCP Service Code Registry maintained by IANA [IANA-SC], or from the
   uncoordinated private space ([RFC4340], 8.1.2.). This value is used
   in the corresponding DCCP-Request packet.

3.2. Using Service Codes in the Network

   Port numbers and IP addresses are the traditional methods to identify
   a flow within an IP network. When the DCCP header has not been
   encrypted, Middleboxes [RFC3234] SHOULD instead use the Service Code
   to identify the application-service (even when running on a non-
   standard port). Middlebox devices are therefore expected to check
   Service Code values before port numbers for DCCP.

   DCCP connections identified by the Service Code continue to use IP
   addresses and ports, although neither port number may be well-
   known/reserved. Network address and port translators, known
   collectively as NATs [RFC2663][RFC2766], not only interpret DCCP
   ports, but may also translate/modify them [RFC2993]. Interpreting
   DCCP Service Codes can reduce the need to correctly interpret port
   numbers, leading to new opportunities for network address and port
   translators. The DCCP Service Code may allow services to be
   identified behind NATs, if NATs are not further extended to translate
   Service Codes.

   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 recommendations are provided:

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   o  A middlebox SHOULD use the Service Code value to assist in
      determining the behaviour to be applied to a packet flow (e.g.
      default keep-alive interval, NAT translation, etc).

   o  A middlebox SHOULD NOT modify the Service Code, unless they also
      change the service that a connection is accessing.

   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 Receiver

   A Service Code is used by the host that receives a DCCP-Request to
   associate a DCCP connection with the corresponding application-
   service. At the server, this association must be explicit, i.e. if
   the connection is accepted, the requested Service Code must have been
   previously associated with the destination port at the server.

   Each active socket MUST be associated with exactly one Service Code.
   Passive sockets MAY, at the discretion of an implementation (section
   3.2), be associated with more than one Service Code; this may let
   multiple applications, or multiple versions of the same application,
   listen on the same port, differentiated by Service Code.

   An implementation:

   o  MUST allow a server to be associated with a Service Code on a
      specified port [RFC4340].

   o  MAY also allow a server to set a Service Code that applies to a
      set of acceptable destination ports [RFC4340].

   o  SHOULD provide a method that informs a server of the Service Code
      value that was selected by an active connection.

   A number of options are presented for servers using passively
   listening sockets.  As an example, consider the four cases that could
   arise when two DCCP server applications listen on the same host:

   o  The simplest case is when the two servers are associated with
      different Service Codes and are bound to different server ports
      (see section 3.3.1).

   o  The two servers may be associated with the same DCCP Service Code,
      but be bound to different server ports (see section 3.3.1).

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   o  The two servers could use different DCCP Service Code values, and
      be bound to the same port.  This is discussed in detail in section

   o  The two servers could attempt to use the same DCCP Service Code
      and bind to the same port.  A DCCP implementation MUST disallow
      this and reset the connection, since there is no way for the DCCP
      host to direct a new connection to the correct server application.

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], 8.1.3).

   When the destination port is bound to a server, the host MUST also
   verify that the port has been associated with the specified Service
   Code. A Service Code of zero MUST only be accepted for servers that
   have no associated Service Code or are explicitly associated with the
   Service Code value of zero. Two cases can occur:

   o  If the receiving host is listening on the specified destination
      port number and the Service Code of the DCCP-Request matches one
      of the Service Codes associated with this 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 port is not associated with the requested Service Code, the
      server MUST reject the request by sending a DCCP-Reset packet with
      Reset Code 8, "Bad Service Code" ([RFC4340], 8.1.2).

   After a connection has been accepted, the protocol control block is
   associated with the pair of ports and the pair of IP addresses and
   one Service Code value.

3.3.2. Multiple Associations of Service Codes and Ports at the Server

   RFC 4340 states that a Service Code MAY be associated with more than
   one destination port (corresponding to a set of port values). Also a
   single destination port MAY 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 port.  This approach can simplify

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   middlebox processing, e.g. it should not be necessary to create more
   than one hole in a firewall for this to be the case; for example DTLS
   connections and unencrypted connections for the same application will
   normally use different Service Codes to distinguish them, but because
   this is the same application, it makes sense to use the same port.

3.3.3. Automatically launching a Server

   A host implementation may permit a service to be associated with a
   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". This may allow a server to offer more than the limit of
   65,536 services determined by the size of the Port field (fewer if
   system/user/dynamic boundaries are preserved). The upper limit is
   based solely on the number of unique connections between two hosts
   (i.e., 4,294,967,296).

   As in the previous section, when the specified Service Code is not
   associated with the specified port, the connection MUST be aborted
   and a DCCP Reset message sent [RFC4340].

4. Benchmarking Services Described in this document

   A number of simple services are commonly supported by systems using
   DCCP and UDP, this section defines corresponding services for DCCP.
   These services are useful to debug and benchmark bidirectional DCCP
   connections. The IANA section of this document allocates a
   corresponding set of code points for these services.

4.1. Echo

   The operation of the DCCP echo service follows that specified for UDP
   [RFC862]: a server listens for DCCP connections; once a client has
   set up a connection, each data packet sent to the server will be
   copied (echoed) back to the client.

4.2. Daytime

   The DCCP daytime service is operationally equivalent to the
   connection-based TCP daytime service [RFC867]: any data received is
   discarded by the server; and generates a response sent in a DCCP data
   packet containing the current time and data as an ASCII string; after
   which the connection is closed.

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4.3. Character generator

   The operation of the DCCP chargen service corresponds to the
   connection-based TCP chargen protocol [RFC864]: A server listens for
   incoming requests and, once a client has established a connection,
   continuously sends datagrams containing a random number (between 0
   and 512, up to the current path MTU) of characters. The service
   terminates when the user either closes or aborts the connection.
   Congestion control is enforced using the mechanisms [RFC4340] and
   related documents.

   If necessary the receiver can enforce flow control on this service by
   using either or both of the Slow Receiver ([RFC4340], 11.6) and Data
   Dropped ([RFC4340], 11.7) options to signal the server to slow down.

   The chargen protocol provides a useful service that may be used for
   testing and measurement of bidirectional DCCP connectivity, as well
   as congestion control responsiveness. The datagram-based variant of
   chargen can be emulated with the DCCP ECHO service by changing the
   format of the datagrams sent by the client, hence these services
   complement each other.

4.4. Time service

   The format of timestamps and the operation of the DCCP time service
   is equivalent with the TCP time protocol variant [RFC868]: a server
   listens for incoming connections; after a client has established a
   new connection, the server sends a 4-byte timestamp; whereupon the
   client closes the connection.

4.5. PerfTest service

   The PerfTest concept specified by this document provides a generic
   service that may be used to benchmark and measure both unidirectional
   and bidirectional DCCP connections, as well as server and host DCCP

   This document defines a generic PerfTest service. The payload of DCCP
   packets associated with the DCCP PerfTest service are silently
   discarded by the receiver, and used only for gathering numerical
   performance data.

   The PerfTest server is identified by a combination of the port number
   and DCCP Service Code. It does not recommend a specific benchmarking
   software to use, but does allocate a port number specified that
   currently coincides with that of the open-source iperf benchmarking
   program [iperf].

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5. Security Considerations

   This document does not describe new protocol functions.

   The document discusses the usage of Service Codes. There are three
   areas of security that are important:

   1. Interaction with NATs and firewalls (see section 3.2, on middlebox

   2. Interpretation of DCCP Service Codes over-riding traditional use
      of reserved/well-known port numbers (see section 8.1)

   3. Interaction with IPsec and DTLS security (see section 8.2, on use
      of IPsec).

   4. Services used for benchmarking and testing may also be used to
      generate traffic for other purposes, and pose an opportunity for a
      Denial of Service attack. Care needs to be exercised when enabling
      these services in an operational network, or appropriate rate-
      limits should be provided to mitigate these effects.

5.1. Interactions of Service Codes and port numbers

   The Service Code value may be used to over-ride the traditional way
   operating systems consider low-numbered ports as privileged. This
   represents a change in the way operating systems respect this range
   of DCCP port numbers.

   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. 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. Different versions of a service
   (application) may also be mapped to a corresponding set of Service
   Code values. Care needs to be exercised when interpreting the mapping
   of a Service Code value to the corresponding service.

   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.

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5.2. Interactions with IPsec

   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.

   This is not an issue for IPsec because the entire DCCP header and
   payload are protected by all IPsec modes. None of the DCCP header is
   protected by application-layer security, e.g., DTLS [ID.DTLS.DCCP],
   so again this is not an issue [RFC4347].

6. IANA Considerations

   A set of new services are defined in section 6 and are summarized in
   this section.

   >>> Author Note: This section requires consideration by the IANA and
   the DCCP WG -                    - issues need to be identified.

   >>> Author Note: Which numbering space should this apply to? -                                                                     - Free-
   allocation may be easier to manage in the dynamic port-space using a
   separate DCCP registry that is independent of TCP, UDP, and other
   IETF-defined transport protocols?

   To encourage application writers to register their applications, and
   to avoid restricting DCCP service codes to a 16-bit space, we revise
   RFC 4340 as follows:

   "IANA should allocate well-known DCCP ports on demand to anyone to
   applies, without requiring a specification or additional
   justification. Each well-known port request MUST be for a specific
   registered DCCP Service Code. The procedure may allow both to be
   assigned in the same request.

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   IANA MUST use an allocation policy that attempts to minimize server
   port collisions, but it is expected that the same well-known port
   will sometimes be allocated to more than one Service Code."

6.1. Port number values allocated by this document

   IANA action is required to assign ports for use by DCCP. This
   document requests allocation of the following code points from the
   IANA DCCP Port numbers registry:

   >>>>>> IANA ACTION Please replace IANA THIS RFC, with the allocated
   RFC  number. <<<

   echo      7/dccp   Echo SC:ECHO
   # IETF dccp WG, [IANA - THIS RFC]
   daytime   13/dccp  DayTime    SC:DTIM
   # IETF dccp WG, [IANA - THIS RFC]
   chatgen   19/dccp  Chargen    SC:CHAR
   # IETF dccp WG, [IANA - THIS RFC]
   time      37/dccp  Timeserver SC:TIME
   # IETF dccp WG, [IANA - THIS RFC]
   perf    5001/dccp  PerfTest   SC:PERF
   # IETF dccp WG, [IANA - THIS RFC]

6.2. Service Code values allocated by this document

   This document solicits IANA action to allocate the following code
   points from the Service Code registry [IANA-SC]. The requested
   assignments are listed below and summarized in table 1. This set of
   Service Codes may be utilized for testing DCCP implementations and
   transmission paths.

   >>> IANA Please replace tbd by the assigned a port number in section

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    | Service  | ASCII|Port|          Description          |   Ref    |
    | Code (SC)| Code |    |                               |          |
    |1162037327| ECHO |   7| Echo service                  | [RFC862] |
    |0x4543484f|      |    |                               |          |
    |1146374477| DTIM |  13| Daytime server                | [RFC867] |
    |0x4454494d|      |    |                               |          |
    |1128808786| CHAR |  19| Character generator (chargen) | [RFC864] |
    |0x43484152|      |    |                               |          |
    |1414090053| TIME |  37| Timeserver                    | [RFC868] |
    |0x54494d45|      |    |                               |          |
    |1346720326| PERF |5001| Performance tests (e.g.       | *        |
    |0x50455246|      |    | iperf, ttcp, ...)             |          |
     Table 1: Allocation of Service Codes by this document.

     1)  Port is the default port associated with this service.
     2)  * Reference is this document.

   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 (e.g.

7. 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, and the DCCP WG for helpful
   comments on this topic, and Gerrit Renker for his help in determining
   DCCP behaviour, review of the document, and compilation of useful
   test applications defined in the IANA section of this document. Mark
   Handley provided significant input to 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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8. References

8.1. Normative References

   [RFC1122] Braden, R. (ed.), "Requirements for Internet Hosts:
             Communication Layers, " STD 3, RFC 1122, Oct. 1989

   [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

8.2. Informative References

   [IANA]    Internet Assigned Numbers Authority, www.iana.org

   [IANA-SC] IANA DCCP Service Code Registry

   [ID.Portnames] J. Touch, "A TCP Option for Port Names", IETF Work in
             Progress, draft-touch-tcp-portnames-00.txt.

   [ID.DTLS.DCCP] T.Phelan, "Datagram Transport Layer Security (DTLS)
             over the Datagram Congestion Control Protocol (DCCP)", IETF
             Work in Progress, draft-phelan-dccp-dtls-xx.txt.

   [ID.DCCP.RTP] C. Perkins, "RTP and the Datagram Congestion Control
             Protocol (DCCP)", IETF Work in Progress, draft-ietf-dccp-

   [ID.TSVWG.RAND] M. Larsen, F. Gont, "Port Randomization", IETF Work
             in Progress, draft-larsen-tsvwg-port-randomization-00.

   [iperf]   http://dast.nlanr.net/Projects/Iperf/

   [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).

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   [RFC862]  Postel, J., "Echo Protocol", STD 20, RFC 862, May 1983.

   [RFC864]  Postel, J., "Character Generator Protocol", STD 22, RFC
             864, May 1983.

   [RFC867]  Postel, J., "Daytime Protocol", STD 25, RFC 867, May 1983.

   [RFC868]   Postel, J. and K. Harrenstien, "Time Protocol", STD 26,
             RFC 868, May 1983.

   [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
             Translator (NAT) Terminology and Considerations", RFC 2663,
             August 1999.

   [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address Translation
             - Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
             Values In the Internet Protocol and Related Headers", BCP
             37, RFC 2780, March 2000.

   [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
             Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
             L., and V. Paxson, "Stream Control Transmission Protocol",
             RFC 2960, October 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.

   [RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
             Stevens, "Basic Socket Interface Extensions for IPv6", RFC
             3493, February 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.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, December 2005.

   [RFC4347] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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9. Author's Addresses

   Godred (Gorry) Fairhurst
   Department of Engineering
   University of Aberdeen
   Kings College
   Aberdeen, AB24 3UE

   Email: gorry@erg.abdn.ac.uk
   URL:   http://www.erg.abdn.ac.uk/users/gorry

9.1. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

9.2. Disclaimer of Validity

   This document and the information contained herein are provided on

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9.3. 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.

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APPENDIX A: API support for Service Codes

   A potential issue in defining an API for DCCP arises when an
   application binds to a port it needs to specify the associated DCCP
   Service Code. This requires an API that allows a service to be
   associated with a Service Code in addition to a port number. One
   approach is to use separate commands as follows:

   o  Extend the existing port number indicator command (e.g., Unix
      bind() or connect() calls) to also select a specific Service Code
      where desired.

   o  Extend the existing socket parameterization command (e.g., Unix
      setsockopt()) to set a service-code option. This is implemented in
      the present Linux API for a DCCP socket (where the Service Code
      should be wrapped by htonl/ntohl to ensure network byte order).

   o  An information base (table) may be used by servers to identify the
      set of Service Codes that are associated with each port and the
      corresponding set of server applications.

   The current socket API generally requires separate requests to bind
   the port and to set the Service Code for the socket.  This is not a
   problem, providing that an implementation requires both to be
   specified before the socket is allowed to accept connections.

   The host API SHOULD provide a method that returns the Service code of
   an incoming connection request to the application. This may be used
   by an application to correctly process a connection that arrives at a
   port for which it has registered more than one Service Code.

   >>> Author note:

   May need to discuss:

   get_port_and_service_code_by_name(char *what_service_do_you_want)

   char *get_service_code_by_number(unsigned sc)

   and interactions with getadddrinfo() address/port lookup routine,
   which has been introduced to simplify the migration to IPv6
   ([RFC3493], 6.1).

   Functions such as getnameinfo and getservent may also need to be
   updated. >>> End Author Note.

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   >>> 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.


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   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 -                              - no longer required.

   Draft was reorganized to improve clarity and simplify concepts.


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