Datagram Convergence Layers for the DTN Bundle and LTP Protocols
draft-irtf-dtnrg-dgram-clayer-02

DTNRG                                                           H. Kruse
Internet-Draft                                                   S. Jero
Intended status: Experimental                               S. Ostermann
Expires: August 5, 2013                                  Ohio University
                                                                Feb 2013

    Datagram Convergence Layers for the DTN Bundle and LTP Protocols
                    draft-irtf-dtnrg-dgram-clayer-02

Abstract

   This document is a product of the DTNRG Research Group.  It specifies
   the preferred method for transporting DTN protocol data over the
   Internet using datagrams.  The specification covers convergence
   layers for the Bundle Protocol as well as the transportation of LTP
   segments.  UDP and 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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 5, 2013.

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

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

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1.  Introduction

   Delay/Disruption Tolerant Network (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 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.  This document represents the consensus of the DTNRG
   RG.

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
   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 in directly in datagrams
   and using UDP or the Datagram Congestion Control Protocol (DCCP) are
   not generally appropriate because they offer limited reliability and,
   in the case of UDP, no congestion control.

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   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 provides 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

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 options
   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

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   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.

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.
   Bundles that are too large for the path MTU SHOULD be fragmented 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.

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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
   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

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   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.

4.  IANA Considerations

   Port number assignments 1113/UDP and 4556/UDP have been registered
   with IANA.  Assigned port numbers are 1113/DCCP for the transport of
   LTP, and 4556/DCCP for the transport of Bundles.  Assigned DCCP
   Service Codes are 7107696 for tunneling LTP and 1685351985 for
   tunneling Bundle Protocol.

5.  Security Considerations

   This memo describes the use of datagrams to transport DTN application

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   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.

   [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.

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   [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.

   [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,

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              November 2008.

Authors' Addresses

   Hans Kruse
   Ohio University
   292 Lindley Hall
   Athens, OH  45701
   United States

   Phone: +1 740 593 4891
   Email: kruse@ohiou.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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