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RFC 3639 - Considerations on the use of a Service Identifier in


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Network Working Group                                  M. St. Johns, Ed.
Request for Comments: 3639                                G. Huston, Ed.
Category: Informational                                              IAB
                                                            October 2003

                    Considerations on the use of a
                  Service Identifier in Packet Headers

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This memo describes some considerations relating to the use of IP
   protocol number fields and payload protocol (e.g., TCP) port fields
   to identify particular services that may be associated with that port
   number or protocol number.

1.  Introduction

   This memo describes some considerations relating to the use of IP
   protocol number fields and payload protocol (e.g., TCP) port or
   service fields to identify particular services that may be associated
   with that port number or protocol number.  It is a general statement
   regarding appropriate processing and use of service identifiers by
   intermediate systems.

   This memo points out that various measures by intermediate systems
   that are intended to filter or prevent the transmission of traffic
   based on the service identification within the traffic flow will have
   a limited effect.   This will also have a major side-effect of
   forcing the affected services to be redesigned using various forms of
   encapsulation or dynamic port negotiation in order to remove the
   fixed service identification from the IP packet headers.  The IAB
   does not believe this serves the general interests of the Internet
   community related to the design of simple and reliable Internet
   applications.  This memo suggests some thought be given to control
   mechanisms that do not rely on intermediary systems taking actions
   based on an assumed relationship between the service identifier in
   the packet and the actual service of which the packet is a part.

2.  Service Identifiers

   Although not necessarily by design, certain conventions have evolved
   with respect to the IP protocol suite relative to the identification
   of services within an IP traffic flow:

   o  Within the IP protocol suite, end point identifiers (e.g.,
      TCP/UDP/SCTP port numbers, IP protocol numbers) are designed to
      identify services to end points.  In particular, TCP, UDP or SCTP
      (Stream Control Transmission Protocol) port numbers are intended
      to identify the source service location and the destination
      service entity to the destination end point.

   o  The IP [2] datagram header contains the source and destination
      address of the datagram as well as an indication of the upper-
      level protocol (ULP) carried within the datagram.  If the ULP is
      either TCP [3], UDP [1], or SCTP [8] the payload will contain both
      source and destination port numbers which allows differentiation
      between services (e.g., TELNET, HTTP) and between multiple
      instances of the same service between the pair of hosts described
      by the source and destination address.

   o  By convention, for at least TCP and UDP, certain port numbers are
      used as rendezvous points and are considered "well known" on the
      source or destination side of the communication.  Such rendezvous
      points are maintained in an IANA registry currently located at
      [11].  Specific registries for protocol and port numbers are at
      [12] and [13].

   o  Notwithstanding the "well knownness" of any given port, port
      numbers are only guaranteed to be meaningful to the end systems.
      An intermediate system should generally not impute specific
      meaning to any given port number, unless specifically indicated by
      an end system (e.g., via the Resource Reservation Protocol (RSVP)
      [4]) or agreed to by convention among the end systems and one or
      more specific intermediate systems (e.g., firewall traversal for
      the IP Security Protocol (IPSEC) [5]).

   o  Some services make use of protocol interactions to dynamically
      allocate service identifiers (i.e., port numbers) to specific
      communications.  One specific example of this is the Session
      Initiation Protocol (SIP) [9].  The implication of this is that
      intermediate systems cannot relate the service identifiers to the
      actual service unless they participate in the protocols which
      allocate the service identifiers, or are explicitly notified of
      the outcome of the allocation.

   o  Various products and service-related mechanisms deployed today
      take advantage of the fact that some service identifiers are
      relatively stable (and well known) to do various things (e.g.,
      firewall filtering, QOS marking).

   o  Certain network operations, such as various forms of packet
      encapsulation (e.g., tunneling) and encryption, can occlude this
      port number (or service identifier) while an IP packet is in
      transit within the network.  For example, both the IPSEC
      Encapsulating Security Payload (ESP) [6] and Generic Routing
      Encapsulation (GRE) [7] both provide means for tunneling an IP
      datagram within another IP datagram.  The service information
      becomes obscured and, in some instances, encrypted.

   o  Cooperating end systems may elect to use arbitrarily selected port
      numbers for any service.  The port numbers used in such cases may
      be statically defined, through coordinated configuration of the
      cooperating end systems through use of a common application or
      operating system, or by dynamic selection as an outcome of a
      rendezvous protocol.

   Intermediate system imposed service-based controls may block
   legitimate uses by subscribers.  For example, some service providers
   are blocking port 25 (i.e., notionally SMTP) traffic for the stated
   purpose of trying to prevent SPAM, but which can also block
   legitimate email to the end user.

   Attempts by intermediate systems to impose service-based controls on
   communications against the perceived interests of the end parties to
   the communication are often circumvented [10].  Services may be
   tunneled within other services, proxied by a collaborating external
   host (e.g., an anonymous redirector), or simply run over an alternate
   port (e.g., port 8080 vs port 80 for HTTP).  Another means of
   circumvention is alteration of the service behavior to use a dynamic
   port negotiation phase, in order to avoid use of a constant port
   address.

   For the purposes of this memo, a "party to a communication" is either
   the sender, receiver, or an authorized agent of the sender or
   receiver in the path.

   If intermediate systems take actions on behalf of one or more parties
   to the communication or affecting the communication, a good rule of
   thumb is they should only take actions that are beneficial to or
   approved by one or more of the parties, within the operational
   parameters of the service-specific protocol, or otherwise unlikely to
   lead to widespread evasion by the user community.

3.  Ramifications

   The IAB observes that having stable and globally meaningful service
   identifiers visible at points other than the end systems can be
   useful for the purposes of determining network behavior and network
   loading on a macro level.  The IAB also observes that application
   protocols that include dynamic port negotiation for both ends of a
   connection tend to add to the complexity of the applications.

   Dynamic port negotiation for a protocol may also limit or prohibit
   its use in situations where the service provider (e.g., ISP or
   employer) has instituted some form of service filtering through port
   blocking mechanisms.

   From this perspective of network and application utility, it is
   preferable that no action or activity be undertaken by any agency,
   carrier, service provider, or organization which would cause end-
   users and protocol designers to generally obscure service
   identification information from the IP packet header.

   Nothing in this statement should be construed as opposing
   encapsulation, application security, end-to-end encryption, or other
   processes beneficial or specifically desired by the end-users.

4.  Security Considerations

   This document is a general statement regarding appropriate processing
   and use of service identifiers by intermediate systems.  If enough
   agencies, carriers, service providers, and organizations ignore the
   concerns voiced here, the utility of port and protocol numbers,
   general network analysis, end-user beneficial filtering (e.g.,
   preventing DDOS attacks), and other common uses of these service
   identifiers might be adversely affected.

5.  References

   [1]   Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

   [2]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
         1981.

   [3]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

   [4]   Braden, B., Ed., Zhang, L., Berson, S., Herzog, S. and S.
         Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
         Functional Specification", RFC 2205, September 1997.

   [5]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [6]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

   [7]   Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
         "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

   [8]   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.

   [9]   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.

   [10]  New York Times, "STUDENTS EVADE UNIVERSITY TACTICS TO PROTECT
         MEDIA FILES", 27th November 2002.

   [11]  IANA, "IANA Protocol Numbers and Assignment Services", May
         2003, <http://www.iana.org/numbers.htm>.

   [12]  IANA, "IANA Protocol Number Registry", May 2003, <http://
         www.iana.org/assignments/protocol-numbers>.

   [13]  IANA, "IANA Port Number Registry", May 2003, <http://
         www.iana.org/assignments/port-numbers>.

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Appendix A. IAB Members

   Internet Architecture Board Members at the time this document was
   completed were:

   Bernard Aboba
   Harald Alvestrand
   Rob Austein
   Leslie Daigle, Chair
   Patrik Faltstrom
   Sally Floyd
   Jun-ichiro Itojun Hagino
   Mark Handley
   Geoff Huston
   Charlie Kaufman
   James Kempf
   Eric Rescorla
   Michael St Johns

Editors' Addresses

   Mike St Johns
   Internet Architecture Board

   EMail: mstjohns@mindspring.com

   Geoff Huston
   Internet Architecture Board

   EMail: gih@telstra.net

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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