CoRE Working Group C. Bormann
Internet-Draft K. Hartke
Intended status: Informational Universitaet Bremen TZI
Expires: February 25, 2011 August 24, 2010
Miscellaneous additions to CoAP
draft-bormann-coap-misc-06
Abstract
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 February 25, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. A Compact Accept Option . . . . . . . . . . . . . . . . . . . 4
3. URI encoding . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Stateful URI compression . . . . . . . . . . . . . . . . . 6
3.2. Support for URIs with Binary Adresses . . . . . . . . . . 7
3.3. Where are Uri Options Used? . . . . . . . . . . . . . . . 8
4. Option Encoding . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. A More Efficient Option Encoding . . . . . . . . . . . . . 10
4.2. Critical Options . . . . . . . . . . . . . . . . . . . . . 11
4.3. Errors in Options . . . . . . . . . . . . . . . . . . . . 12
4.4. Payload-Length Option . . . . . . . . . . . . . . . . . . 12
4.5. User-defined Option . . . . . . . . . . . . . . . . . . . 13
4.6. Making the Etag option useful . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6.1. Amplification Attacks . . . . . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Things we won't do . . . . . . . . . . . . . . . . . 21
A.1. An efficient stateless URI encoding . . . . . . . . . . . 21
A.2. Media-Types with Self-Describing Length . . . . . . . . . 22
Appendix B. Experimental Options . . . . . . . . . . . . . . . . 24
B.1. Options indicating absolute time . . . . . . . . . . . . . 24
Appendix C. Rationale: Representing Durations . . . . . . . . . . 26
C.1. Text for section 3.2.1 of coap-01 . . . . . . . . . . . . 26
C.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 27
C.3. Pseudo-Floating Point . . . . . . . . . . . . . . . . . . 27
C.4. A Duration Type for CoAP . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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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
channel,
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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14 [RFC2119]
and indicate requirement levels for compliant CoAP implementations.
The term "byte" is used in its now customary sense as a synonym for
"octet".
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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
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. 6, in Table 2
of [I-D.ietf-core-coap]. (Alternatively, as Accept appears to be
mutually exclusive with Content-Type, the same option number as
Content-Type could be used. For both to have the same structure, the
Content-Type option could also be repeated for every Acceptable media
type.)
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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Proposal: Add an Accept Option.
Benefits: A Server does not need to specify one URI each for every
possible media type that it wants to serve a resource under.
(See also Appendix A.2.)
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3. 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
/.well-known/resources
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.
Appendix A.1 defines a potential URI encoding that is slightly more
efficient than the abovementioned ones. However, even that was
rejected by the WG for its unconvincing cost-benefit ratio, which
then went on to discuss Henning Schulzrinne's proposal to add state.
3.1. 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
state.
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 2.
0
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| duration | TeRI...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Option for offering a TeRI in a response
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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
duration.
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.
Proposal: Add a TeRI option (e.g., number 2) that can be used in
CoAP requests and responses.
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.
3.2. Support for URIs with Binary Adresses
As defined in [I-D.ietf-core-coap], the Uri option does not provide a
way to distinguish an "absolute-URI" from an "absolute-path"
[RFC3986]: the leading slash is omitted from the latter. (Ticket
#12.)
The proposal to fix this is to split the option into three parts:
"Uri-Scheme" for the URI Scheme, "Uri-Authority" for Host/Port and
"Uri-Path" for the absolute-path.
A further problem with that version is that the authority part can be
very bulky if it encodes an IPv6 address in ASCII.
Proposal: Provide an option "Uri-Authority-Binary" that can be an
even number of bytes between 2 and 18 except 12 or 14.
o If the number of bytes is 2, the destination IP address of the
packet transporting the CoAP message is implied.
o If the number of bytes is 4 or 6, the first four bytes of the
option value are an IPv4 address in binary.
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o If the number of bytes is 8 or 10, the first eight bytes are the
lower 64 bits of an IPv6 address; the upper eight bytes are
implied from the destination address of the packet transporting
the CoAP message.
o If the number of bytes is 16 or 18, the first 16 bytes are an IPv6
address.
o If two more bytes remain, this is a port number (as always in
network byte order).
The resulting authority is (conceptually translated into ASCII and)
used in place of an Uri-Authority option. Examples:
+-------+--------------------+----------+---------------------------+
| Uri- | Uri-Authority-Bina | Uri-Path | URI |
| Schem | ry | | |
| e | | | |
+-------+--------------------+----------+---------------------------+
| (none | (none) | (none) | "/" |
| ) | | | |
| | | | |
| (none | (none) | 'temp' | "/temp" |
| ) | | | |
| | | | |
| (none | 2 bytes: 61616 | 'temp' | "coap://[DA]:61616/temp" |
| ) | | | |
| | | | |
| 'http | 10 bytes: ::123:45 | (none) | "http://[DA::123:45]:616" |
| ' | + 616 | | |
| | | | |
| (none | 16 bytes: 2000::1 | temp | "coap://[2000::1]/temp" |
| ) | | | |
| | | | |
| 'http | 18 bytes: 2000::1 | temp | "http://[2000::1]:616/tem |
| ' | + 616 | | p" |
+-------+--------------------+----------+---------------------------+
3.3. Where are Uri Options Used?
A URI, represented by an appropriate set of Uri options (one or more
of Uri-Scheme, Uri-Authority, Uri-Path, unless the default absolute-
path of "/" applies) should be present in every GET, PUT, POST, or
DELETE message, unless replaced by a TeRI. The receiver of an ACK
can figure out which URI the message pertains to by comparing
transaction IDs.
In a delayed Confirmable message that contains an asynchronous
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response to a GET, the URI is included again ("echoed back").
In addition to the context provided by including the URI again in
delayed responses, we propose adding a "Token" option, which can be
included in GET, PUT, POST, DELETE messages, and is echoed back in
exactly those cases where the URI is echoed back. The Token allows a
recipient of an asynchronous message to relate back to an earlier
request and thus can be used as the long-term equivalent of what TID
is for a single transaction. The Token option value is a sequence of
bytes, which is opaque to the server. The option is critical.
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4. Option Encoding
The option encoding in [I-D.ietf-core-coap] is neither particularly
flexible nor particularly efficient. One important, easily
overlooked disadvantage of the encoding is the large number of ways
in which the same information can be encoded. This unneeded
variability causes problems in interoperability and increases both
coding and testing efforts required.
4.1. A More Efficient Option Encoding
The basic idea of the proposed encoding is to reduce the number of
ways the same information can be encoded as far as possible (but not
further). This both simplifies decoding (e.g., an implementation
that only ever uses short URIs never has to implement long options,
because these can only be used with long lengths) and
interoperability testing (there is only one way to say a specific
thing, so there aren't multiple ways that need testing).
One of the undesired variations in packets is the ordering of the
options. In this draft, we therefore mandate a total ordering of
options, ordered by the option number.
As an interesting consequence, the option numbers can now be
expressed in delta coding, in turn requiring fewer bits to encode the
option number. This frees a number of bits for the length, making
the likelihood of actually needing the two-byte form of the option
header much smaller.
To further reduce variation, the length of the value (as always, not
including the option header) is now encoded in such a way that there
is only one way to express a given length: The short form (one-byte
option tag) can express length values from 0 to 14, and the long form
is used for values of 15 to 15+255=270, inclusively (Figure 3).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| option delta | length | for 0..14
+---+---+---+---+---+---+---+---+
for 15..270:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| option delta | 1 1 1 1 | length - 15 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 3: Option delta/length representation with small range
The small option delta of 0..15 in this encoding limits the
difference in option value between two adjacent options (or the value
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of the option number of the first option). While realistic sequences
of options rarely will have a problem here, option numbers 14, 28,
... are reserved for no-op options with no body (implementations will
automatically ignore these with zero additional code; see Section 4.2
why the reserved values are not 15, 30, ...). Note that the
resulting delta that reaches the interim nop option may have any
number, e.g., for including option 2 and 27 in one message, the
sequence would be:
o delta = 2 (option 2, body)
o delta = 12 (option 14 = no-op, no body)
o delta = 13 (option 27, body)
In the unlikely case that only option 40 is needed, the sequence
would be:
o delta = 14 (option 14 = no-op, no body)
o delta = 14 (option 28 = no-op, bo body)
o delta = 12 (option 40, body)
4.2. Critical Options
CoAP is designed to enable the definition of additional options by
later extensions. Typically, new options are designed in such a way
that they can simply be ignored if not understood, i.e. new options
are _elective_. However, some new options may be _critical_, i.e.,
there is no good way to process the message if the option is not
understood. (Actually, half of the options currently on the table
are critical in nature.)
In the option encoding proposed, odd-numbered options indicate a
critical option; even-numbered options indicate elective options.
(Note that, again, the even/odd distinction is on the option number
resulting from the decoding, not the delta value actually embedded in
the packet.)
Implementing this proposal requires some renumbering of options from
[I-D.ietf-core-coap].
Note that most options that are designated as critical are _not_
meant to be mandatory to implement. "Critical" just means if such an
option is encountered in an incoming message, there is no meaningful
way to process the message unless it is indeed implemented.
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4.3. Errors in Options
When a message contains a critical option that is not understood by
the receiver, we say that _decoding fails_.
When a message contains an option that is defined for a specific
length of value (e.g., Max-Age, which is only defined for length 1),
this is treated like an unknown option. For a critical option, this
causes the decoding to fail. For an elective option, this is not an
error, the option with the unsupported structure is just ignored.
(In both cases, the intention is to allow extension of the option by
different syntax in a later revision of the protocol.)
If the decoding of a message fails, the processing depends on the
message type:
o NCN messages and RST messages with decode failures are always
silently ignored.
o CON messages with decode failures lead to an RST with error code
400 (Bad Request). The payload of the RST SHOULD be a copy of the
option bytes that caused decoding to fail. However, nodes with
minimal capabilities may choose to restrict their error processing
to a minimum,
o ACK messages that fail to decode are hard to process. The
requesting node MAY repeat the request with fewer options in order
to receive a simpler answer; if that is not possible, the decoding
failure should be treated like a client error. Conversely, nodes
SHOULD not send critical options in ACK messages unless the CON
message eliciting the ACK contained options that justify this.
(There may be exceptions, e.g., a node is always allowed to send a
Block option with a large resource representation. A requestor
that does not understand Block may never be able to receive that
resource representation properly, so it is appropriate to treat
the situation as a client error.)
4.4. Payload-Length Option
Not all transport mappings may provide an unambiguous length of the
CoAP message. For UDP, it may also be desirable to pack more than
one CoAP message into one UDP payload (aggregation); in that case,
for all but the last message there needs to be a way to delimit the
payload of that message.
We propose a new option, the Payload-Length option. If this option
is present, the value of this option is an unsigned integer giving
the length of the payload of the message (note that this integer can
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be zero for a zero-length payload, which can in turn be represented
by a zero-length option value). (In the UDP aggregation case, what
would have been in the payload of this message after "payload-length"
bytes is then actually one or more additional messages.)
4.5. User-defined Option
To enable experimentation and community-specific options, option
number 14 (the first NOP option) can also be used as a user-defined
option. For this application, the option value has one or more
bytes, the semantics of which are defined by prior agreement between
the communicating partners.
It is RECOMMENDED to start the option value with a unique identifier,
e.g., the SDNV [RFC5050] of the enterprise number of the organisation
defining the option, possibly followed by additional discriminating
bits or bytes.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0 0 0 0 0 0 1|0 1 1 1 0 0 1 1| private opt-id| value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\___SDNV of enterprise number___/
Figure 4: Example option value for user-defined option
User-defined Options are elective.
The User-defined Option is repeatable.
4.6. Making the Etag option useful
Problem: The Etag option only allows for up to four bytes in one
Etag. If Etags are computed with a random distribution (e.g., by
hashing the resource representation), the birthday paradox makes a
collision surprisingly likely already for 1e4 variants in
circulation.
Proposal: Allow longer Etags (i.e., don't specify a specific upper
limit). The default Apache Etag has about 8..12 Bytes of
information in it (file ID = inode number, size, timestamp; which
interestingly is mostly redundant with information available in
Content-Length and Last-Modified). If a tighter upper limit is
desired, 8 Bytes should suffice for all practical purposes, but
makes two-way gatewaying with HTTP more complex.
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Problem: The Etag option is not useful without an equivalent of
"If-None-Match".
Proposal: Make Etags useful by defining a new Option "I-Have": The
I-Have Option carries a variable length octet sequence that
specifies the Etag of a resource representation that is being
cached by a client. A client that has one or more representations
previously obtained from the resource can indicate to the server
that it has cached these representations, such that the server may
omit the representation when the state of the resource does not
change from a cached representation or changes to one of the other
cached representation. The I-Have Option may occur zero or more
times. (This can be used to represent a "If-None-Match" HTTP
option with one or more Etags. Note that CoAP always uses what
would be called a strong validator in HTTP.) As it appears I-Have
and Etag are mutually exclusive, I-Have can use the option number
of Etag.
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5. IANA Considerations
This draft adds option numbers to Table 2 of [I-D.ietf-core-coap],
resulting in:
+-----+----+------------------+--------------+-------+--------------+
| Typ | C/ | Name | Data type | Lengt | Default |
| e | E | | | h | |
+-----+----+------------------+--------------+-------+--------------+
| 0 | E | TeRI | Duration + | 2-n B | (none) |
| | | | Sequence of | | |
| | | | Bytes | | |
| | | | | | |
| 1 | C | Content-type | Unsigned | 1 B | 0 |
| | | | Integer | | (text/plain) |
| | | | | | |
| 2 | E | Max-age | Duration | 1 B | 0 |
| | | | | | |
| 3 | C | Uri-Scheme | String | 1-n B | 'coap' |
| | | | | | |
| 4 | E | Etag | Sequence of | 1-n* | (none) |
| | | | Bytes | B | |
| | | | | | |
| 5 | C | Uri-Authority | String | 1-n B | '' |
| | | | | | |
| 6 | E | Location | String | 1-270 | - |
| | | | | B | |
| | | | | | |
| 7 | C | Uri-Authority-Bi | Sequence of | 1-n B | (use |
| | | nary | Bytes | | Uri-Authorit |
| | | | | | y) |
| | | | | | |
| 8 | E | Accept | Sequence of | 1-n B | any |
| | | | Bytes | | |
| | | | | | |
| 10 | E | Date | Unsigned | 4-6 B | (none) |
| | | | Integer | | |
| | | | (Appendix B. | | |
| | | | 1) | | |
| | | | | | |
| 9 | C | Uri-Path | String | 1-n B | '' |
| | | | | | |
| 11 | C | Token | Sequence of | 1-n B | (none) |
| | | | Bytes | | |
| | | | | | |
| 13 | C | Block (now in | Unsigned | 1-3 B | 0 |
| | | coap-block) | Integer | | |
| | | | | | |
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| 14. | E | Nop | None | 0 B | ('') |
| . | | | | | |
| | | | | | |
| 14 | E | User | Sequence of | 1-n B | (none) |
| | | | Bytes | | |
| | | | | | |
| 15 | C | Payload-length | Unsigned | 0-2 B | (none) |
| | | | Integer | | |
+-----+----+------------------+--------------+-------+--------------+
(The upper limit of "n" indicates that the size is limited only by
the options encoding. * indicates that this document proposes to
change the limit.) Odd option numbers indicate critical options,
even option numbers indicate elective options. Option numbers 14,
28, 42, ... (any number divisible by 14) are reserved (they are
elective and therefore ignored by all implementations).
(Subscription-related options are discussed in
[I-D.hartke-coap-observe], so the following option from
[I-D.ietf-core-coap] is not further discussed here:)
+------+-----+-----------------------+-----------+--------+---------+
| Type | C/E | Name | Data type | Length | Default |
+------+-----+-----------------------+-----------+--------+---------+
| 10 | E | Subscription-lifetime | Duration | 1 B | 0 |
+------+-----+-----------------------+-----------+--------+---------+
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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.)
6.1. Amplification Attacks
TBD. (This section discusses how CoAP nodes could become implicated
in DoS attacks by using the amplifying properties of the protocol, as
well as mitigations for this threat.)
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7. Acknowledgements
This work was partially funded by the Klaus Tschira Foundation.
Of course, much of the content of this draft is the result of
discussions with the [I-D.ietf-core-coap] authors. Tokens were
suggested by Gilman Tolle.
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8. References
8.1. Normative References
[I-D.hartke-coap-observe]
Hartke, K. and C. Bormann, "Observing Resources in CoAP",
draft-hartke-coap-observe-01 (work in progress),
July 2010.
[I-D.ietf-core-coap]
Shelby, Z., Frank, B., and D. Sturek, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-01
(work in progress), July 2010.
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 1: URIs, Connections, and
Message Parsing", draft-ietf-httpbis-p1-messaging-11 (work
in progress), August 2010.
[I-D.ietf-httpbis-p4-conditional]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 4: Conditional Requests",
draft-ietf-httpbis-p4-conditional-11 (work in progress),
August 2010.
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y.,
Nottingham, M., and J. Reschke, "HTTP/1.1, part 6:
Caching", draft-ietf-httpbis-p6-cache-11 (work in
progress), August 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.
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8.2. 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.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
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Appendix A. Things we won't do
This annex documents roads that the WG decided not to take, in order
to spare readers from reinventing them in vain.
A.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
frequency:
%.aeinorst
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 5.
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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 5: 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 6. 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
experiment.)
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 6: Nibble-based URI encoding: 21 -> 15 bytes
A.2. Media-Types with Self-Describing Length
Open Issues: For coap-00, the Accept option proposed would have
needed a way to handle two-byte media types (easiest if these can
be made self-describing, at the cost of about 3 bits in the sub-
type field; Figure 7).
An self-describing representation for long mediatypes could look like
this:
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0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| top | sub | (1-byte: unchanged)
+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 000 | top | sub | (2-byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: A self-describing media type representation
Instead, we assume for now that CoAP-01 will switch to a single-byte
media type encoding.
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Appendix B. Experimental Options
This annex documents proposals that need significant additional
discussion before they can become part of (or go back to) the main
CoAP specification. They are not dead, but might die if there turns
out to be no good way to solve the problem.
B.1. Options indicating absolute time
HTTP has a number of headers that may indicate absolute time:
o "Date", defined in Section 14.18 in [RFC2616] (Section 9.3 in
[I-D.ietf-httpbis-p1-messaging]), giving the absolute time a
response was generated;
o "Last-Modified", defined in Section 14.29 in [RFC2616], (Section
6.6 in [I-D.ietf-httpbis-p4-conditional], giving the absolute time
of when the origin server believes the resource representation was
last modified;
o "If-Modified-Since", defined in Section 14.25 in [RFC2616],
"If-Unmodified-Since", defined in Section 14.28 in [RFC2616], and
"If-Range", defined in Section 14.27 in [RFC2616] can be used to
supply absolute time to gate a conditional request;
o "Expires", defined in Section 14.21 in [RFC2616] (Section 3.3 in
[I-D.ietf-httpbis-p6-cache]), giving the absolute time after which
a response is considered stale.
o The more obscure headers "Retry-After", defined in Section 14.37
in [RFC2616], and "Warning", defined in section 14.46 in
[RFC2616], also may employ absolute time.
[I-D.ietf-core-coap] defines a single "Date" option, which however
"indicates the creation time and date of a given resource
representation", i.e., is closer to a "Last-Modified" HTTP header.
HTTP's caching rules [I-D.ietf-httpbis-p6-cache] make use of both
"Date" and "Last-Modified", combined with "Expires". The specific
semantics required for CoAP needs further consideration.
In addition to the definition of the semantics, an encoding for
absolute times needs to be specified.
In UNIX-related systems, it is customary to indicate absolute time as
an integer number of seconds, after midnight UTC, January 1, 1970.
Unless negative numbers are employed, this time format cannot
represent time values prior to January 1, 1970, which probably is not
required for the uses ob absolute time in CoAP.
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If a 32-bit integer is used and allowance is made for a sign-bit in a
local implementation, the latest UTC time value that can be
represented by the resulting 31 bit integer value is 03:14:07 on
January 19, 2038. If the 32-bit integer is used as an unsigned
value, the last date is 2106-02-07, 06:28:15.
The reach can be extended by: - moving the epoch forward, e.g. by 40
years (= 1262304000 seconds) to 2010-01-01. This makes it impossible
to represent Last-Modified times in that past (such as could be
gatewayed in from HTTP). - extending the number of bits, e.g. by one
more byte, either always or as one of two formats, keeping the 32-bit
variant as well.
Also, the resolution can be extended by expressing time in
milliseconds etc., requiring even more bits (e.g., a 48-bit unsigned
integer of milliseconds would last well after year 9999.)
For experiments, an experimental "Date" option is defined with the
semantics of HTTP's "Last-Modified". It can carry an unsigned
integer of 32, 40, or 48 bits; 32- and 40-bit integers indicate the
absolute time in seconds since 1970-01-01 00:00 UTC, while 48-bit
integers indicate the absolute time in milliseconds since 1970-01-01
00:00 UTC.
However, that option is not really that useful until there is a
"If-Modified-Since" option as well.
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Appendix C. Rationale: Representing Durations
C.1. Text for section 3.2.1 of coap-01
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. CoAP durations represent integer numbers of
seconds, but instead of representing these numbers as integers, a
more compact single-byte pseudo-floating-point (pseudo-FP)
representation is used (Figure 8).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0... value |
+---+---+---+---+---+---+---+---+
+---+---+---+---+---+---+---+---+
| 1... mantissa | exponent |
+---+---+---+---+---+---+---+---+
Figure 8: Duration in (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, with the exponent field cleared out but
still present) 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 definitions), as demonstrated in Figure 9.
#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 9: Decoding an (8,4) pseudo-FP value
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.
The highest pseudo-FP value, represented by an all-ones byte (0xFF),
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is reserved to indicate an indefinite duration. The next lower value
(0xEF) is thus the highest representable value and is decoded as
7340032 seconds, a little more than 12 weeks.
C.2. Rationale
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
represented?
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.
C.3. 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
slow.
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
representation.
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These issues have led to protocols that try to constrain themselves
to integer representation even where the ability of a floating point
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
libraries.
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 8 illustrates an (8,4)-pseudo-FP value.
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 9.
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.
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).
C.4. 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
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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. Figure 10 shows the values that can be expressed. (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 seconds.)
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.
It might be worthwhile to reserve one duration value, e.g. 0xFF, for
an indefinite duration.
Duration Seconds Encoded
----------- ---------- -------
00:00:00 0x00000000 0x00
00:00:01 0x00000001 0x01
00:00:02 0x00000002 0x02
00:00:03 0x00000003 0x03
00:00:04 0x00000004 0x04
00:00:05 0x00000005 0x05
00:00:06 0x00000006 0x06
00:00:07 0x00000007 0x07
00:00:08 0x00000008 0x08
00:00:09 0x00000009 0x09
00:00:10 0x0000000a 0x0a
00:00:11 0x0000000b 0x0b
00:00:12 0x0000000c 0x0c
00:00:13 0x0000000d 0x0d
00:00:14 0x0000000e 0x0e
00:00:15 0x0000000f 0x0f
00:00:16 0x00000010 0x10
00:00:17 0x00000011 0x11
00:00:18 0x00000012 0x12
00:00:19 0x00000013 0x13
00:00:20 0x00000014 0x14
00:00:21 0x00000015 0x15
00:00:22 0x00000016 0x16
00:00:23 0x00000017 0x17
00:00:24 0x00000018 0x18
00:00:25 0x00000019 0x19
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00:00:26 0x0000001a 0x1a
00:00:27 0x0000001b 0x1b
00:00:28 0x0000001c 0x1c
00:00:29 0x0000001d 0x1d
00:00:30 0x0000001e 0x1e
00:00:31 0x0000001f 0x1f
00:00:32 0x00000020 0x20
00:00:33 0x00000021 0x21
00:00:34 0x00000022 0x22
00:00:35 0x00000023 0x23
00:00:36 0x00000024 0x24
00:00:37 0x00000025 0x25
00:00:38 0x00000026 0x26
00:00:39 0x00000027 0x27
00:00:40 0x00000028 0x28
00:00:41 0x00000029 0x29
00:00:42 0x0000002a 0x2a
00:00:43 0x0000002b 0x2b
00:00:44 0x0000002c 0x2c
00:00:45 0x0000002d 0x2d
00:00:46 0x0000002e 0x2e
00:00:47 0x0000002f 0x2f
00:00:48 0x00000030 0x30
00:00:49 0x00000031 0x31
00:00:50 0x00000032 0x32
00:00:51 0x00000033 0x33
00:00:52 0x00000034 0x34
00:00:53 0x00000035 0x35
00:00:54 0x00000036 0x36
00:00:55 0x00000037 0x37
00:00:56 0x00000038 0x38
00:00:57 0x00000039 0x39
00:00:58 0x0000003a 0x3a
00:00:59 0x0000003b 0x3b
00:01:00 0x0000003c 0x3c
00:01:01 0x0000003d 0x3d
00:01:02 0x0000003e 0x3e
00:01:03 0x0000003f 0x3f
00:01:04 0x00000040 0x40
00:01:05 0x00000041 0x41
00:01:06 0x00000042 0x42
00:01:07 0x00000043 0x43
00:01:08 0x00000044 0x44
00:01:09 0x00000045 0x45
00:01:10 0x00000046 0x46
00:01:11 0x00000047 0x47
00:01:12 0x00000048 0x48
00:01:13 0x00000049 0x49
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00:01:14 0x0000004a 0x4a
00:01:15 0x0000004b 0x4b
00:01:16 0x0000004c 0x4c
00:01:17 0x0000004d 0x4d
00:01:18 0x0000004e 0x4e
00:01:19 0x0000004f 0x4f
00:01:20 0x00000050 0x50
00:01:21 0x00000051 0x51
00:01:22 0x00000052 0x52
00:01:23 0x00000053 0x53
00:01:24 0x00000054 0x54
00:01:25 0x00000055 0x55
00:01:26 0x00000056 0x56
00:01:27 0x00000057 0x57
00:01:28 0x00000058 0x58
00:01:29 0x00000059 0x59
00:01:30 0x0000005a 0x5a
00:01:31 0x0000005b 0x5b
00:01:32 0x0000005c 0x5c
00:01:33 0x0000005d 0x5d
00:01:34 0x0000005e 0x5e
00:01:35 0x0000005f 0x5f
00:01:36 0x00000060 0x60
00:01:37 0x00000061 0x61
00:01:38 0x00000062 0x62
00:01:39 0x00000063 0x63
00:01:40 0x00000064 0x64
00:01:41 0x00000065 0x65
00:01:42 0x00000066 0x66
00:01:43 0x00000067 0x67
00:01:44 0x00000068 0x68
00:01:45 0x00000069 0x69
00:01:46 0x0000006a 0x6a
00:01:47 0x0000006b 0x6b
00:01:48 0x0000006c 0x6c
00:01:49 0x0000006d 0x6d
00:01:50 0x0000006e 0x6e
00:01:51 0x0000006f 0x6f
00:01:52 0x00000070 0x70
00:01:53 0x00000071 0x71
00:01:54 0x00000072 0x72
00:01:55 0x00000073 0x73
00:01:56 0x00000074 0x74
00:01:57 0x00000075 0x75
00:01:58 0x00000076 0x76
00:01:59 0x00000077 0x77
00:02:00 0x00000078 0x78
00:02:01 0x00000079 0x79
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00:02:02 0x0000007a 0x7a
00:02:03 0x0000007b 0x7b
00:02:04 0x0000007c 0x7c
00:02:05 0x0000007d 0x7d
00:02:06 0x0000007e 0x7e
00:02:07 0x0000007f 0x7f
00:02:08 0x00000080 0x80
00:02:24 0x00000090 0x90
00:02:40 0x000000a0 0xa0
00:02:56 0x000000b0 0xb0
00:03:12 0x000000c0 0xc0
00:03:28 0x000000d0 0xd0
00:03:44 0x000000e0 0xe0
00:04:00 0x000000f0 0xf0
00:04:16 0x00000100 0x81
00:04:48 0x00000120 0x91
00:05:20 0x00000140 0xa1
00:05:52 0x00000160 0xb1
00:06:24 0x00000180 0xc1
00:06:56 0x000001a0 0xd1
00:07:28 0x000001c0 0xe1
00:08:00 0x000001e0 0xf1
00:08:32 0x00000200 0x82
00:09:36 0x00000240 0x92
00:10:40 0x00000280 0xa2
00:11:44 0x000002c0 0xb2
00:12:48 0x00000300 0xc2
00:13:52 0x00000340 0xd2
00:14:56 0x00000380 0xe2
00:16:00 0x000003c0 0xf2
00:17:04 0x00000400 0x83
00:19:12 0x00000480 0x93
00:21:20 0x00000500 0xa3
00:23:28 0x00000580 0xb3
00:25:36 0x00000600 0xc3
00:27:44 0x00000680 0xd3
00:29:52 0x00000700 0xe3
00:32:00 0x00000780 0xf3
00:34:08 0x00000800 0x84
00:38:24 0x00000900 0x94
00:42:40 0x00000a00 0xa4
00:46:56 0x00000b00 0xb4
00:51:12 0x00000c00 0xc4
00:55:28 0x00000d00 0xd4
00:59:44 0x00000e00 0xe4
01:04:00 0x00000f00 0xf4
01:08:16 0x00001000 0x85
01:16:48 0x00001200 0x95
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01:25:20 0x00001400 0xa5
01:33:52 0x00001600 0xb5
01:42:24 0x00001800 0xc5
01:50:56 0x00001a00 0xd5
01:59:28 0x00001c00 0xe5
02:08:00 0x00001e00 0xf5
02:16:32 0x00002000 0x86
02:33:36 0x00002400 0x96
02:50:40 0x00002800 0xa6
03:07:44 0x00002c00 0xb6
03:24:48 0x00003000 0xc6
03:41:52 0x00003400 0xd6
03:58:56 0x00003800 0xe6
04:16:00 0x00003c00 0xf6
04:33:04 0x00004000 0x87
05:07:12 0x00004800 0x97
05:41:20 0x00005000 0xa7
06:15:28 0x00005800 0xb7
06:49:36 0x00006000 0xc7
07:23:44 0x00006800 0xd7
07:57:52 0x00007000 0xe7
08:32:00 0x00007800 0xf7
09:06:08 0x00008000 0x88
10:14:24 0x00009000 0x98
11:22:40 0x0000a000 0xa8
12:30:56 0x0000b000 0xb8
13:39:12 0x0000c000 0xc8
14:47:28 0x0000d000 0xd8
15:55:44 0x0000e000 0xe8
17:04:00 0x0000f000 0xf8
18:12:16 0x00010000 0x89
20:28:48 0x00012000 0x99
22:45:20 0x00014000 0xa9
1d 01:01:52 0x00016000 0xb9
1d 03:18:24 0x00018000 0xc9
1d 05:34:56 0x0001a000 0xd9
1d 07:51:28 0x0001c000 0xe9
1d 10:08:00 0x0001e000 0xf9
1d 12:24:32 0x00020000 0x8a
1d 16:57:36 0x00024000 0x9a
1d 21:30:40 0x00028000 0xaa
2d 02:03:44 0x0002c000 0xba
2d 06:36:48 0x00030000 0xca
2d 11:09:52 0x00034000 0xda
2d 15:42:56 0x00038000 0xea
2d 20:16:00 0x0003c000 0xfa
3d 00:49:04 0x00040000 0x8b
3d 09:55:12 0x00048000 0x9b
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3d 19:01:20 0x00050000 0xab
4d 04:07:28 0x00058000 0xbb
4d 13:13:36 0x00060000 0xcb
4d 22:19:44 0x00068000 0xdb
5d 07:25:52 0x00070000 0xeb
5d 16:32:00 0x00078000 0xfb
6d 01:38:08 0x00080000 0x8c
6d 19:50:24 0x00090000 0x9c
7d 14:02:40 0x000a0000 0xac
8d 08:14:56 0x000b0000 0xbc
9d 02:27:12 0x000c0000 0xcc
9d 20:39:28 0x000d0000 0xdc
10d 14:51:44 0x000e0000 0xec
11d 09:04:00 0x000f0000 0xfc
12d 03:16:16 0x00100000 0x8d
13d 15:40:48 0x00120000 0x9d
15d 04:05:20 0x00140000 0xad
16d 16:29:52 0x00160000 0xbd
18d 04:54:24 0x00180000 0xcd
19d 17:18:56 0x001a0000 0xdd
21d 05:43:28 0x001c0000 0xed
22d 18:08:00 0x001e0000 0xfd
24d 06:32:32 0x00200000 0x8e
27d 07:21:36 0x00240000 0x9e
30d 08:10:40 0x00280000 0xae
33d 08:59:44 0x002c0000 0xbe
36d 09:48:48 0x00300000 0xce
39d 10:37:52 0x00340000 0xde
42d 11:26:56 0x00380000 0xee
45d 12:16:00 0x003c0000 0xfe
48d 13:05:04 0x00400000 0x8f
54d 14:43:12 0x00480000 0x9f
60d 16:21:20 0x00500000 0xaf
66d 17:59:28 0x00580000 0xbf
72d 19:37:36 0x00600000 0xcf
78d 21:15:44 0x00680000 0xdf
84d 22:53:52 0x00700000 0xef
91d 00:32:00 0x00780000 0xff (reserved)
Figure 10
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Fax: +49-421-218-7000
Email: hartke@tzi.org
Bormann & Hartke Expires February 25, 2011 [Page 35]