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Versions: 00 03 04 05 RFC 7122

DTNRG                                                           H. Kruse
Internet-Draft                                                   S. Jero
Intended status: Experimental                               S. Ostermann
Expires: April 20, 2014                                  Ohio University
                                                        October 17, 2013


    Datagram Convergence Layers for the DTN Bundle and LTP Protocols
                    draft-irtf-dtnrg-dgram-clayer-05

Abstract

   This document is a product of the Delay- and Distruption-Tolerant
   Networking Research Group (DTNRG), and represents the consensus of
   the DTNRG.  It specifies the preferred method for transporting Delay-
   and Disruption-Tolerant Networking (DTN) protocol data over the
   Internet using datagrams.  The specification covers convergence
   layers for the Bundle Protocol (RFC 5050) as well as the
   transportation of Licklider Transmission Protocol (LTP) (RFC 5326)
   segments.  UDP and the Datagram Congestion Control Protocol (DCCP)
   are the candidate datagram protocols discussed.  UDP can only be used
   on a local network, or in cases where the DTN node implements
   explicit congestion control.  DCCP addresses the congestion control
   problem, and its use is recommended whenever possible.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on April 20, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  General Recommendation  . . . . . . . . . . . . . . . . . . .   4
   3.  Recommendations for Implementers  . . . . . . . . . . . . . .   5
     3.1.  How and Where to Deal with Fragmentation  . . . . . . . .   6
       3.1.1.  DCCP  . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Bundle Protocol over a Datagram Convergence Layer . . . .   6
       3.2.1.  DCCP  . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  LTP over Datagrams  . . . . . . . . . . . . . . . . . . .   7
       3.3.1.  DCCP  . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.3.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Keep Alive Option . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Checksums . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.5.1.  DCCP  . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.5.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  DCCP Congestion Control Modules . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   DTN communication protocols include the Bundle Protocol described in
   RFC 5050 [RFC5050], which provides transmission of application data
   blocks (Bundles) through optional intermediate custody transfer, and
   the Licklider Transmission Protocol (LTP), RFCs 5325 (LTP Motivation)
   [RFC5325], 5326 (LTP Specification) [RFC5326], and 5327 (LTP
   Security) [RFC5327] which can be used to transmit Bundles reliably
   and efficiently over a point to point link.  It is often desirable to
   test these protocols over Internet Protocol links.  draft-irtf-dtnrg-
   tcp-clayer [I-D.irtf-dtnrg-tcp-clayer] defines a method for



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   transporting Bundles over TCP.  This draft specifies the preferred
   method for transmitting either Bundles or LTP blocks across the
   Internet using datagrams in place of TCP.  Figure 1 shows the general
   protocol layering described in the DTN documents.  DTN Applications
   interact with the Bundle Protocol Layer, which in turn uses a
   Convergence Layer to prepare a bundle for transmission.  The
   Convergence Layer will typically rely on a lower level protocol to
   carry out the transmission.


                    +-----------------------------------------+
                    |                                         |
                    |             DTN Application             |
                    |                                         |
                    +-----------------------------------------+
                    +-----------------------------------------+
                    |                                         |
                    |           Bundle Protocol (BP)          |
                    |                                         |
                    +-----------------------------------------+
                    +-----------------------------------------+
                    |                                         |
                    |      Convergence Layer Adapter (CL)     |
                    |                                         |
                    +-----------------------------------------+
                    +-----------------------------------------+
                    |                                         |
                    |    Local Data Link Layer (Transport)    |
                    |                                         |
                    +-----------------------------------------+


                 Figure 1: Generic Protocol Stack for DTN

   This document provides guidance for implementation of the two
   protocol stacks illustrated in figure 2.  In figure 2(a) the
   Convergence Layer Adapater is UDP or DCCP for direct transport of
   Bundles over the Internet.  In figure 2(b) the Convergence Layer
   Adapter is LTP, which then uses UDP or DCCP as the local data link
   layer.











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                    +-------------+         +-------------+
                    |             |         |             |
                    |   DTN App   |         |   DTN App   |
                    |             |         |             |
                    +-------------+         +-------------+
                    +-------------+         +-------------+
                    |             |         |             |
                    |      BP     |         |      BP     |
                    |             |         |             |
                    +-------------+         +-------------+
                    +-------------+         +-------------+
                    |             |         |             |
                    |  UDP/DCCP   |         |      LTP    |
                    |             |         |             |
                    +-------------+         +-------------+
                                            +-------------+
                                            |             |
                                            |  UDP/DCCP   |
                                            |             |
                                            +-------------+

                          (a)                     (b)


           Figure 2: Protocol Stacks Addressed in this Document

1.1.  Requirements Language

   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.  General Recommendation

   In order to utilize DTN protocols across the Internet, whether for
   testing purposes or as part of a larger network path, it is necessary
   to encapsulate them into a standard Internet protocol so that they
   travel easily across the Internet.  This is particularly true for
   LTP, which provides no endpoint addressing.  This encapsulation
   choice needs to be made carefully in order to avoid redundancy, since
   DTN protocols may provide their own reliability mechanisms.

   Congestion control is vital to the continued functioning of the
   Internet, particularly for situations where data will be sent at
   arbitrarily fast data rates.  The Bundle Protocol delegates provision
   of reliable delivery and, implicitly, congestion control to the
   convergence layer used (Section 7.2 of RFC 5050 [RFC5050]).  In
   situations where TCP will work effectively in communications between



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   pairs of DTN nodes, use of the TCP convergence layer draft-irtf-
   dtnrg-tcp-clayer [I-D.irtf-dtnrg-tcp-clayer] will provide the
   required reliability and congestion control for transport of Bundles
   and would be the default choice in the Internet.  Alternatives such
   as encapsulating Bundles directly in datagrams and using UDP or DCCP
   are not generally appropriate because they offer limited reliability
   and, in the case of UDP, no congestion control.

   LTP, on the other hand, offers its own form of reliability.
   Particularly for testing purposes, it makes no sense to run LTP over
   a protocol, like TCP, that offers reliability already.  In addition,
   running LTP over TCP would reduce the flexibility available to users,
   since LTP offers more control over what data is delivered reliably
   and what data is delivered best effort, a feature that TCP lacks.  As
   such, it would be better to run LTP over an unreliable protocol.

   One solution would be to use UDP.  UDP provides no reliability,
   allowing LTP to manage that itself.  However, UDP does not provide
   congestion control.  Because LTP is designed to run over fixed rate
   radio links it does provide rate control, but not congestion control.
   Lack of congestion control in network connections is a major problem
   that can cause artificially high loss rates and/or serious fairness
   issues.  Previous standards documents are unanimous in recommending
   congestion control for protocols to be used on the Internet, see
   "Congestion Control Principles" [RFC2914], "Unicast UDP Usage
   Guidelines" [RFC5405], and "Queue Management and Congestion
   Avoidance" [RFC2309], among others.  RFC 5405, in particular, calls
   congestion control "vital" for "applications that can operate at
   higher, potentially unbounded data rates".  Therefore, any Bundle
   Protocol implementation permitting the use of UDP to transport LTP
   segments or Bundles outside an isolated network for the transmission
   of any non-trivial amounts of data MUST implement congestion control
   consistent with RFC 5405.

   Alternatively, the Datagram Congestion Control Protocol (DCCP)
   [RFC4340] was designed specifically to provide congestion control
   without reliability for those applications that traverse the Internet
   but do not desire to retransmit lost data.  As such, it is
   RECOMMENDED that, if possible, DCCP be used to transport LTP segments
   across the Internet.

3.  Recommendations for Implementers









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3.1.  How and Where to Deal with Fragmentation

   The Bundle Protocol allows Bundles with sizes limited only by node
   resource constraints.  In IPv4, the maximum size of a UDP datagram is
   nearly 64KB.  In IPv6, when using jumbograms [RFC2675], UDP datagrams
   can technically be up to 4GB in size [RFC2147], although this option
   is rarely used.  It is well understood that sending large IP
   datagrams that must be fragmented by the network has enormous
   efficiency penalties [Kent88].  The Bundle Protocol specification
   provides a Bundle fragmentation concept [RFC5050] that allows a large
   Bundle to be divided into Bundle fragments.  If the Bundle Protocol
   is being encapsulated in DCCP or UDP, it therefore SHOULD create each
   fragment of sufficiently small size that it can then be encapsulated
   into a datagram that will not need to be fragmented at the IP layer.

   IP fragmentation can be avoided by using IP Path MTU Discovery
   [RFC1191][RFC1981], which depends on the deterministic delivery of
   ICMP Packet Too Big (PTB) messages from routers in the network.  To
   bypass a condition referred to as a black hole [RFC2923], a newer
   specification is available in [RFC4821] to determine the IP Path MTU
   without the use of PTB messages.  This document does not attempt to
   recommend one fragmentation avoidance mechanism over another; the
   information in this section is included for the benefit of
   implementers.

3.1.1.  DCCP

   Because DCCP implementations are not required to support IP
   fragmentation and are not allowed to enable it by default, a DCCP
   Convergence Layer (we will use "CL" from here on) MUST NOT accept
   data segments that cannot be sent as one MTU sized datagram.

3.1.2.  UDP

   When an LTP CL is using UDP for datagram delivery, it SHOULD NOT
   create segments that will result in UDP datagrams that will need to
   be fragmented, as discussed above.

3.2.  Bundle Protocol over a Datagram Convergence Layer

   In general, the use of the Bundle Protocol over a datagram CL is
   discouraged in IP networks.  Bundles can be of (almost) arbitrary
   length, and the Bundle Protocol does not include an effective
   retransmission mechanism.  Whenever possible the Bundle Protocol
   SHOULD be operated over the TCP Convergence Layer or over LTP.

   If a datagram CL is used for transmission of Bundles, every datagram
   MUST contain exactly one Bundle or four zero octets as a keep-alive.



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   Bundles that are too large for the path MTU SHOULD be fragmented and
   reassembled at the Bundle Protocol layer to prevent IP fragmentation.

3.2.1.  DCCP

   The DCCP CL for Bundle Protocol use SHOULD use the IANA assigned port
   4556/DCCP and service code 1685351985; the use of other port numbers
   and service codes is implementation specific.

3.2.2.  UDP

   The UDP CL for Bundle Protocol use SHOULD use the IANA assigned port
   4556/UDP; the use of other port numbers is implementation specific.

3.3.  LTP over Datagrams

   LTP is designed as a point to point protocol within DTN, and it
   provides intrinsic acknowledgement and retransmission facilities.
   LTP segments are transported over a "local data link layer" (RFC 5325
   [RFC5325]); we will use the term "transport" from here on.  Transport
   of LTP using datagrams is an appropriate choice.  When a datagram
   transport is used to send LTP segments, every datagram MUST contain
   exactly one LTP segment or four zero octets as a keep-alive.  LTP
   MUST perform segmentation in such a way as to ensure that every LTP
   segment fits into a single packet which will not require IP
   fragmentation as discussed above.

3.3.1.  DCCP

   The DCCP transport for LTP SHOULD use the IANA assigned port 1113/
   DCCP and service code 7107696; the use of other port numbers and
   service codes is implementation specific.

3.3.2.  UDP

   The UDP transport for LTP SHOULD use the IANA assigned port 1113/UDP;
   the use of other port numbers is implementation specific.

3.4.  Keep Alive Option

   It may be desirable for a UDP or DCCP CL or transport to send "keep-
   alive" packets during extended idle periods.  This may be needed to
   refresh a contact table entry at the destination, or to maintain an
   address mapping in a NAT or a dynamic access rule in a firewall.
   Therefore, the CL or transport MAY send a datagram containing exactly
   4 octets of zero bits.  The CL or transport receiving such a packet
   MUST discard this packet; the receiving CL or transport may then
   perform local maintenance of its state tables, these maintenance



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   functions are not covered in this draft.  Note that packets carrying
   Bundles or segments will always contain more than 4 octets of
   information (either the Bundle or the LTP header); keep-alive packets
   will therefore never be mistaken for actual data packets.  If UDP or
   DCCP are being used for communication in both directions between a
   pair of Bundle agents, transmission and processing of keep-alives in
   the two directions occurs independently.  Keep-alive intervals SHOULD
   be configurable, SHOULD default to 15 sec, and MUST NOT be configured
   shorter than 15 sec.

3.5.  Checksums

   Both the core Bundle Protocol specification and core LTP
   specification assume that they are transmitting over an erasure
   channel, i.e. a channel that either delivers packets correctly or not
   at all.

3.5.1.  DCCP

   A DCCP transmitter MUST, therefore, ensure that the entire packet is
   checksummed by setting the Checksum Coverage to 0.  Likewise, the
   DCCP receiver MUST ignore all packets with partial checksum coverage.

3.5.2.  UDP

   A UDP transmitter therefore MUST NOT disable UDP checksums, and the
   UDP receiver MUST NOT disable checking of received UDP checksums.

   Even when UDP checksums are enabled a small probability of UDP packet
   corruption remains.  In some environments it may be acceptable for
   LTP or the Bundle Protocol to occasionally receive corrupted input.
   In general, however, a UDP implementation SHOULD use optional
   security extensions available in the Bundle Protocol or LTP to
   protect against message corruption.

3.6.  DCCP Congestion Control Modules

   DCCP supports pluggable congestion control modules in order to
   optimize its behavior to particular environments.  The two most
   common congestion control modules (CCIDs) are TCP-like Congestion
   Control (CCID2) [RFC4341] and TCP-Friendly Rate Control (CCID3)
   [RFC4342].  TCP-like Congestion Control is designed to emulate TCP's
   congestion control as much as possible.  It is recommended for
   applications that want to send data as quickly as possible, while
   TCP-Friendly Rate Control is aimed at applications that want to avoid
   sudden changes in sending rate.  DTN use cases seem to fit more into
   the first case so DCCP CL's and transports SHOULD use TCP-like
   Congestion Control (CCID2) by default.



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4.  IANA Considerations

   Port number assignments 1113/UDP and 4556/UDP have been registered
   with IANA.  Port number 1113/DCCP with Service Code 7107696 has been
   requested for the transport of LTP under IANA ticket number 717731.
   Port number 4556/DCCP with Service Code 1685351985 has been requested
   for the transport of Bundles under IANA ticket number 717732.

5.  Security Considerations

   This memo describes the use of datagrams to transport DTN application
   data.  Hosts may be in the position of having to accept and process
   packets from unknown sources; the DTN Endpoint ID can be discovered
   only after the Bundle has been retrieved from the DCCP or UDP packet.
   Hosts SHOULD use authentication methods available in the DTN
   specifications to prevent malicious hosts from inserting unknown data
   into the application.

   Hosts need to listen for and process DCCP or UDP data on the known
   LTP or Bundle Protocol ports.  A denial of service scenario exists
   where a malicious host sends datagrams at a high rate, forcing the
   receiving hosts to use their resources to process and attempt to
   authenticate this data.  Whenever possible, hosts SHOULD use IP
   address filtering to limit the origin of packets to known hosts.

6.  References

6.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, December 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2147]  Borman, D., "TCP and UDP over IPv6 Jumbograms", RFC 2147,
              May 1997.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.





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   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, March 2006.

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC5325]  Burleigh, S., Ramadas, M., and S. Farrell, "Licklider
              Transmission Protocol - Motivation", RFC 5325, September
              2008.

   [RFC5326]  Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
              Transmission Protocol - Specification", RFC 5326,
              September 2008.

   [RFC5327]  Farrell, S., Ramadas, M., and S. Burleigh, "Licklider
              Transmission Protocol - Security Extensions", RFC 5327,
              September 2008.

6.2.  Informative References

   [I-D.irtf-dtnrg-tcp-clayer]
              Demmer, M., Ott, J., and S. Perreault, "Delay Tolerant
              Networking TCP Convergence Layer Protocol", draft-irtf-
              dtnrg-tcp-clayer-05 (work in progress), January 2013.

   [Kent88]   Kent, C. and J. Mogul, "Fragmentation considered
              harmful.", 1988, <http://doi.acm.org/10.1145/55482.55524>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
              2923, September 2000.




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   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              March 2006.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November
              2008.

Authors' Addresses

   Hans Kruse
   Ohio University
   31 S. Court Street, Rm 150
   Athens, OH  45701
   United States

   Phone: +1 740 593 4891
   Email: kruse@ohio.edu


   Samuel Jero
   Ohio University
   Athens, Ohio  45701
   United States

   Email: sj323707@ohio.edu


   Shawn Ostermann
   Ohio University
   Stocker Engineering Center
   Athens, OH  45701
   United States

   Phone: +1 740 593 1566
   Email: ostermann@eecs.ohiou.edu











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