CoRE Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                             June 25, 2010
Expires: December 27, 2010

                    Miscellaneous additions to CoAP


   This short I-D makes a number of partially interrelated proposals how
   to solve certain problems in the CoRE WG's main protocol, CoAP.

Status of this Memo

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

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   This Internet-Draft will expire on December 27, 2010.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  A Compact Accept Option  . . . . . . . . . . . . . . . . . . .  4
   3.  Representing Durations . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Pseudo-Floating Point  . . . . . . . . . . . . . . . . . .  5
     3.2.  A Duration Type for CoAP . . . . . . . . . . . . . . . . .  7
   4.  URI encoding . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  An efficient stateless URI encoding  . . . . . . . . . . .  8
     4.2.  Stateful URI compression . . . . . . . . . . . . . . . . . 10
   5.  Block-wise transfers . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  The Block Option . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   The CoRE WG is tasked with standardizing an Application Protocol for
   Constrained Networks/Nodes, CoAP.  This protocol is intended to
   provide RESTful [REST] services not unlike HTTP [RFC2616], while
   reducing the complexity of implementation as well as the size of
   packets exchanged in order to make these services useful in a highly
   constrained network of themselves highly constrained nodes.

   This objective requires restraint in a number of sometimes
   conflicting ways:

   o  reducing implementation complexity in order to minimize code size,

   o  reducing message sizes in order to minimize the number of
      fragments needed for each message (in turn to maximize the
      probability of delivery of the message), the amount of
      transmission power needed and the loading of the limited-bandwidth

   o  reducing requirements on the environment such as stable storage,
      good sources of randomness or user interaction capabilities.

   This draft attempts to address a number of problems not yet
   adequately solved in [I-D.ietf-core-coap].  The solutions proposed to
   these problems are somewhat interrelated and are therefore presented
   in one draft.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14 [RFC2119]
   and indicate requirement levels for compliant CoAP implementations.

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2.  A Compact Accept Option

   A resource may be available in a number of representations.  Without
   some information from the client, a server has no easy way to decide
   which of these would be best served.  HTTP has an Accept: request
   header that a client can use to indicate the media types supported,
   allowing the server to decide.  This header is somewhat unpopular as,
   for a web browser, there are too many media types to choose from, so
   -- even with wildcards -- there is no meaningful information to put
   there.  (This has changed a bit for AJAX calls, which may indeed have
   a specific media type preference.)  It is unlikely that machine-to-
   machine communication would have the same problem.

   A similar function to the HTTP Accept: header could be added to CoAP
   as an option in a much simpler way.  The CoAP Accept option would
   simple carry a value that is a sequence of octets, each of which is
   an acceptable media type for the client, in the order of preference
   (see Figure 1).  If no Accept option is given, the client does not
   express a preference.

           0 1 2 3 4 5 6 7
          |   mediatype   |

           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          |   mediatype1  |   mediatype2  |    etc.

         Figure 1: Accept option value: A sequence of media types

   Accept also has to be given an option type code, e.g. 7, in Table 2
   of [I-D.ietf-core-coap].

   The other addition that would be required is an error code that
   mirrors HTTP's "415 Unsupported Media Type".  This is indeed already
   listed as CoAP Code 35 in Table 3 of [I-D.ietf-core-coap].

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3.  Representing Durations

   Various message types used in CoAP need the representation of
   *durations*, i.e. of the length of a timespan.  In SI units, these
   are measured in seconds.  Where CPU power and memory is abundant, a
   duration can almost always be adequately represented by a non-
   negative floating-point number representing that number of seconds.
   Historically, many APIs have also used an integer representation,
   which limits both the resolution (e.g., if the integer represents the
   duration in seconds) and often the range (integer machine types have
   range limits that may become relevant).  UNIX's "time_t" (which is
   used for both absolute time and durations) originally was a signed
   32-bit value of seconds, but was later complemented by an additional
   integer to add microsecond ("struct timeval") and then later
   nanosecond ("struct timespec") resolution.

   Three decisions need to be made for each application of the concept
   of duration:

   o  the *resolution*.  What rounding error is acceptable?

   o  the *range*.  What is the maximum duration that needs to be

   o  the *number of bits* that can be expended.

   Obviously, these decisions are interrelated.  Typically, a large
   range needs a large number of bits, unless resolution is traded.  For
   most applications, the actual requirement for resolution are limited
   for longer durations, but can be more acute for shorter durations.

3.1.  Pseudo-Floating Point

   Constrained systems typically avoid the use of floating-point (FP)
   values, as

   o  simple CPUs often don't have support for floating-point datatypes

   o  software floating-point libraries are expensive in code size and

   In addition, floating-point datatypes used to be a significant
   element of market differentiation in CPU design; it has taken the
   industry a long time to agree on a standard floating point

   These issues have led to protocols that try to constrain themselves
   to integer representation even where the ability of a floating point

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   representation to trade range for resolution would be beneficial.

   The idea of introducing _pseudo-FP_ is to obtain the increased range
   provided by embedding an exponent, without necessarily getting stuck
   with hardware datatypes or inefficient software floating-point

   For the purposes of this draft, we define an (n,e)-pseudo-FP as a
   fixed-length value of n bits, e of which may be used for an exponent.
   Figure 2 illustrates an (8,4)-pseudo-FP value.

     0   1   2   3   4   5   6   7
   | 0...          value           |

   | 1... mantissa |    exponent   |

                Figure 2: An (8,4) pseudo-FP representation

   If the high bit is clear, the entire n-bit value (including the high
   bit) is the decoded value.  If the high bit is set, the mantissa
   (including the high bit, but with the exponent field cleared out) is
   shifted left by the exponent to yield the decoded value.

   The (n,e)-pseudo-FP format can be decoded with a single line of code
   (plus a couple of constant definition), as demonstrated in Figure 3.

   #define N 8
   #define E 4
   #define HIBIT (1 << (N - 1))
   #define EMASK ((1 << E) - 1)
   #define MMASK ((1 << N) - 1 - EMASK)

   #define DECODE_8_4(r) (r < HIBIT ? r : (r & MMASK) << (r & EMASK))

                Figure 3: Decoding an (8,4) pseudo-FP value

   Only non-negative numbers can be represented by this format.  It is
   designed to provide full integer resolution for values from 0 to
   2^(n-1)-1, i.e., 0 to 127 in the (8,4) case, and a mantissa of n-e
   bits from 2^(n-1) to (2^n-2^e)*2^(2^e-1), i.e., 128 to 7864320 in the
   (8,4) case.  By choosing e carefully, resolution can be traded
   against range.

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   Note that a pseudo-FP encoder needs to consider rounding; different
   applications of durations may favor rounding up or rounding down the
   value encoded in the message.  This requires a little more than a
   single line of code (which is left as an exercise to the reader, as
   the most efficient expression depends on hardware details).

3.2.  A Duration Type for CoAP

   CoAP needs durations in a number of places.  In [I-D.ietf-core-coap],
   durations occur in the option "Subscription-lifetime" as well as in
   the option "Max-age".  (Note that the option "Date" is not a
   duration, but a point in time.)  Other durations of this kind may be
   added later.

   Most durations relevant to CoAP are best expressed with a minimum
   resolution of one second.  More detailed resolutions are unlikely to
   provide much benefit.

   The range of lifetimes and caching ages are probably best kept below
   the order of magnitude of months.  An (8,4)-pseudo-FP has the maximum
   value of 7864320, which is about 91 days; this appears to be adequate
   for a subscription lifetime and probably even for a maximum cache
   age.  (If a larger range for the latter is indeed desired, an (8,5)-
   pseudo-FP could be used; this would last 15 milleniums, at the cost
   of having only 3 bits of accuracy for values larger than 127

   Proposal:  A single duration type is used throughout CoAP, based on
      an (8,4)-pseudo-FP giving a duration in seconds.

   Benefits:  Implementations can use a single piece of code for
      managing all CoAP-related durations.

      In addition, length information never needs to be managed for
      durations that are embedded in other data structures: All
      durations are expressed by a single byte.

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4.  URI encoding

   In HTTP-based systems, URIs can reach significant lengths.  While
   CoAP-based systems may be able to sidestep the most egregious
   excesses (mostly by simply applying REST principles), a URI such as


   can use up one third of the available payload in a CoAP message
   transported in a single 6LoWPAN packet.  Is there a way to encode
   these URIs in a more efficient way?

   Several proposals have been made on the CoRE mailing list, e.g.
   applying the principle of base64-encoding [RFC4648] in reverse and
   using only 6 bits per character.  However, due to rounding errors and
   occasional characters that are not in the 64-character subset chosen
   to be efficiently encodable, the actual gains are limited.
   Similarly, using 7 bits per character (assuming URIs contain only
   ASCII characters) only gives a best-case gain of 12.5 %, and that
   only in the case the URI is a multiple of 8 characters long.  On the
   other hand, the complexity (and danger of subtle interoperability
   problems) is not entirely trivial.

   We will proceed by first proposing an URI encoding that is slightly
   more efficient than the abovementioned ones, then rejecting even that
   for its unconvincing cost-benefit ratio, and finally taking up
   Henning Schulzrinne's proposal to add state.

4.1.  An efficient stateless URI encoding

   There is very little redundancy by repetition in a typical URI,
   rendering popular compression methods such as LZ77 (as implemented in
   in the widely used DEFLATE algorithm [RFC1951]) rather ineffective.

   For the short, non-repetitive data structures that URIs tend to be,
   efficient stateless compression is pretty much confined to Huffman
   (or, for even more complexity, arithmetic) coding.  The complexity
   can be reduced significantly by moving to n-ary Huffman coding, i.e.,
   optimizing not to the bit level, but to a larger level of
   granularity.  Informal experiments by the author show that a 16ary
   Huffman coding is close to optimal for reasonable URI lengths.  In
   other words, basing the encoding on nibbles (4-bit half-bytes) is
   both nearly optimal and relatively inexpensive to implement.

   The actual letter frequencies that will occur in CoAP URIs are hard
   to predict.  As a stopgap, the author has analyzed an HTTP-based URI
   corpus and found the following characters to occur with high

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   In the encoding proposed, each of these ten highly-compressed
   characters is represented by a single 4-bit nibble.  As the %
   character is used for hexadecimal encoding in URIs, two additional
   nibbles are used to provide the numeric value of the two hexadecimal
   numbers following the % character (the original URI will only be
   properly reconstituted if these are upper-case as they should be
   according to section 2.1 of the URI specification [RFC3986]; the
   encoder can choose to send all three characters in dual-nibble format
   if that matters).  An encoder could also map non-ASCII characters to
   this three-nibble form, even though they are not allowed in URIs.
   This gives compatibility with the %-encoding required by [RFC3986].

   All other characters are represented by both of their nibbles.  The
   resulting sequence of nibbles is reconstituted into a sequence of
   bytes in most-significant-nibble-first order.  Any unused nibble in
   the last byte of an encoding is set to 0.  (Upon decoding, this
   padding can be readily distinguished from another % combination as
   this would require another byte after the last byte.)  The encoding
   is summarized in Figure 4.

     0                                       1
     0   1   2   3   4   5   6   7   8   9   0   1
   |    1, 8-F     |   .aeinorst
   +---+---+---+---+   189ABCDEF

   |      2-7      |      0-F      |   other ASCII

   |       0       |      0-F      |      0-F      |   %HH

                   Figure 4: A nibble-based URI encoding

   An example encoding for "/.well-known/resources" (where the initial
   slash is left out, as proposed for abs-path URIs) is given in
   Figure 5.  While the more than 28 % savings in this example may seem
   just an accident, the HTTP-based corpus indeed shows an average
   savings of about 21.8 %, i.e. the sum of the lengths of the encoded
   version of all URIs in the corpus is about 78.2 % of the sum of the
   length of all URIs.  (The savings should be noticeably higher with a
   more RESTful selection of URIs than was available for this

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        0                          1                             2
        1  2  3  4  5  6  7  8  9  0  1  2  3  4  5  6  7  8  9  0  1
     /  .  w  e  l  l  -  k  n  o  w  n  /  r  e  s  o  u  r  c  e  s

       2e 77 65 6c 6c 2d 6b 6e 6f 77 6e 2f 72 65 73 6f 75 72 63 65 73
       1  77 9  6c 6c 2d 6b b  c  77 b  2f d  9  e  c  75 d  63 9  e
     = 17 79 6c 6c 2d 6b bc 77 b2 fd 9e c7 5d 63 9e

            Figure 5: Nibble-based URI encoding: 21 -> 15 bytes

4.2.  Stateful URI compression

   Is the approximately 25 % average saving achievable with Huffman-
   based URI compression schemes worth the complexity?  Probably not,
   because much higher average savings can be achieved by introducing

   Henning Schulzrinne has proposed for a server to be able to supply a
   shortened URI once a resource has been requested using the full-
   length URI.  Let's call such a shortened referent a _Temporary
   Resource Identifier_, _TeRI_ for short.  This could be expressed by a
   response option as shown in Figure 6.

           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          |    duration   |    TeRI...

            Figure 6: Option for offering a TeRI in a response

   The TeRI offer option indicates that the server promises to offer
   this resources under the TeRI given for at least the time given as
   the duration.  Another TeRI offer can be made later to extend the

   Once a TeRI for a URI is known (and still within its lifetime), the
   client can supply a TeRI instead of a URI in its requests.  The same
   option format as an offer could be used to allow the client to
   indicate how long it believes the TeRI will still be valid (so that
   the server can decide when to update the lifetime duration).  TeRIs
   in requests could be distinguished from URIs e.g. by using a
   different option number.

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   Proposal:  Add a TeRI option that can be used in CoAP requests and

      Add a way to indicate a TeRI and its duration in a link-value.

      Do not add any form of stateless URI encoding.

   Benefits:  Much higher reduction of message size than any stateless
      URI encoding could achieve.

      As the use of TeRIs is entirely optional, minimal complexity nodes
      can get by without implementing them.

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5.  Block-wise transfers

   Not all resource representations will fit into a single link layer
   packet of a constrained network.  Using fragmentation (either at the
   adaptation layer or at the IP layer) to enable the transport of
   larger representations is possible up to the maximum size of a UDP
   datagram, but the fragmentation/reassembly process loads the lower
   layers with conversation state that is better managed in the
   application layer.

   This section proposes options to enable _block-wise_ access to
   resource representations.  The overriding objective is to avoid
   creating conversation state at the server for block-wise GET
   requests.  (It is impossible to fully avoid creating conversation
   state for POST/PUT, if the creation/replacement of resources is to be
   atomic; where that property is not needed, there is no need to create
   server conversation state in this case, either.)  Also,
   implementation of these options is intended to be optional.  (The
   details of which parts of the behavior need to be mandatory to enable
   that optionality still are TBD, see below.)

   The size of the blocks should not be fixed by the protocol.  On the
   other hand, implementation should be as simple as possible.  We
   therefore propose a small range of power-of-two block sizes, from 2^4
   (16) to 2^11 (2048) bytes.  One of these eight values can be encoded
   in three bits (0 for 2^4 to 7 for 2^11 bytes), the "szx" (size
   exponent); the actual block size is then "1 << (szx + 4)".

5.1.  The Block Option

   When a representation is larger than can be comfortably transferred
   in a single UDP datagram, the Block option can be used to indicate a
   block-wise transfer.  Block is a 1-, 2- or 3-byte integer, the four
   least significant bits of which indicate the size and whether the
   current block-wise transfer is the last block being transferred (M or
   "more" bit).  The value divided by sixteen is the number of the block
   currently being transferred, starting from zero, i.e., the current
   transfer is about the "size" bytes starting at "blocknr << (szx +
   4)".  The default value of the Block option is zero, indicating that
   the current block is the first (block number 0) and only (M bit not
   set) block of the transfer; however, there is no explicit size
   implied by this default value.

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           0 1 2 3 4 5 6 7
          |blocknr|M| szx |

           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          |        block nr       |M| szx |

           0                   1                   2
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
          |                block nr               |M| szx |

                          Figure 7: Block option

   (Note that the option with the last 4 bits masked out, shifted to the
   left by the value of szx, gives the byte position of the block.  The
   author is not too sure whether that particularly is a feature.)

   The block option is used in one of three roles:

   o  In the request for a GET, it gives the block number requested and
      suggests a block size (block number 0) or echoes the block size of
      previous blocks received (block numbers other than 0).

   o  In the response for a GET or in the request for a PUT or POST, it
      describes what block number is contained in the payload, and
      whether further blocks are part of that body (M bit).  If the M
      bit is set, the size of the payload body in bytes MUST indeed be
      the power of two given by the block size.  All blocks for a
      transaction MUST use the same block size, except for the last
      block (M bit not set).

   o  In the response for a PUT or POST, it indicates what block number
      is being acknowledged.  In this case, the M bit is set to indicate
      that this response does not carry the final response to the
      request; this can occur when the M bit was set in the request and
      the server implements PUT/POST atomically (only with the receptin
      of the last block).

   In all cases, the block number logically extends the transaction ID,
   i.e. the same transaction ID can be used for all exchanges for a
   block-wise transfer.  (For GET, and for PUT/POST where atomic

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   semantics are not needed, the requester is free to use different
   transactions IDs for each block if desired.)

   When a GET is answered with a response carrying a Block option with
   the M bit set, the requestor may retrieve additional blocks by
   sending requests with a Block option giving the block number desired.
   In such a Block option, the M bit MUST be sent as zero and ignored on

   To influence the block size used in response to a GET request, the
   requestor uses the Block option, giving the desired size, a block
   number of zero and an M bit of zero.  A server SHOULD use the block
   size indicated or a smaller size.  Any further block-wise requests
   for blocks beyond the first one MUST indicate the block size used in
   the response for the first one.

   If the Block option is used by the requestor, all GET requests in a
   single transaction MUST use the same size.  The server SHOULD use the
   block size indicated in the request option, but the requestor MUST
   take note of the actual block size used in the response; the server
   MUST ensure that it uses the same block size for all responses in a
   transaction (except for the last one with the M bit not set).  [TBD:
   decide whether the Block option can only be used in a response if a
   Block option was in the request.  Such a minimal block option could
   be of length zero, i.e., would occupy just one byte for the type/
   length information, but is a bit redundant, so it would be nice to
   leave this requirement out, but then every GET requestor has the
   burden of having to cope with receiving Block options.]

   Block-wise transfers SHOULD be used in conjunction with the Etag
   option, unless the representation being exchanged is entirely static
   (not changing over time at all, such as in a schema describing a
   device).  When reassembling the representation from the blocks being
   exchanged, the reassembler MUST compare Etag options.  If the Etag
   options do not match in a GET transfer, the requestor has the option
   of attempting to retrieve fresh values for the blocks it retrieved
   first.  To minimize the resulting inefficiency, the server MAY cache
   the current value of a representation for an ongoing transaction, but
   there is no requirement for the server to establish any state.  The
   server may offer a TeRI with the initial block to reduce the size of
   further block-wise GET requests; this TeRI MAY be short-lived and
   specific to the version of the representation being retrieved (which
   would in effect freeze the representation of the resource
   specifically for the purposes of this block-wise transfer).

   In a PUT or POST transfer, the block option refers to the body in the
   request, i.e., there is no way to perform a block-wise retrieval of
   the body of the response.  Servers that do need to supply large

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   bodies in response to PUT/POST SHOULD therefore be employing
   redirects, possibly offering a TeRI.

   In a PUT or POST transfer that is intended to be implemented in an
   atomic fashion at the server, the actual creation/replacement takes
   place at the time a block with the M bit unset is received.  If not
   all previous blocks are available at the server at this time, the
   transfer fails and error code 4__ (TBD) MUST be returned.  The error
   code 4__ can also be returned at any time by a server that does not
   currently have the resources to store blocks for a block-wise PUT or
   POST transfer that it would intend to implement in an atomic fashion.
   [TBD: a way for a server to derive the equivalent of an Etag for the
   request body, so that when these do not match in a PUT or POST
   transfer, the reassembler MUST discard older blocks.  For now, the
   transaction ID will have to suffice.]

   Proposal:  Add a Block option that can be used for block-wise

   Benefits:  Transfers larger than can be accommodated in constrained-
      network link-layer packets can be performed in smaller blocks.

      No hard-to-manage conversation state is created at the adaptation
      layer or IP layer for fragmentation.

      The transfer of each block is acknowledged, enabling
      retransmission if required.

      Both sides have a say in the block size that actually will be

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6.  Security Considerations

   TBD.  (Weigh the security implications of application layer block-
   wise transfer against those of adaptation-layer or IP-layer
   fragmentation.  Discuss the implications of TeRIs.  Also: Discuss
   nodes without clocks.)

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7.  References

7.1.  Normative References

              Shelby, Z., Frank, B., and D. Sturek, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-00
              (work in progress), June 2010.

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

7.2.  Informative References

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", 2000.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

Bormann                 Expires December 27, 2010              [Page 17]

Internet-Draft                  CoAP-misc                      June 2010

Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921
   Fax:   +49-421-218-7000

Bormann                 Expires December 27, 2010              [Page 18]