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CommandInterest » History » Version 2

Yingdi Yu, 05/25/2015 02:09 PM

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CommandInterest
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===============
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**Command Interest** is an extension of [[SignedInterest|Signed Interest]].
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As a signed interest, a command interest encodes the SignatureInfo and SignatureValue into the last two components of the Interest name.
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    +-------------+----------+-----------------------------------------------------------------------------------+
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    |  Interest   | Interest | +------+--------+--------------------------------------------------+ +----------+ |
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    | Type (0x01) |  length  | | Name |  Name  | +---------+--   --+---------+---------+---------+| | Other    | |
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    |             |          | | Type | Length | |Component|  ...  |Component|Component|Component|| | TLVs ... | |
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    |             |          | |      |        | |  TLV 1  |       | TLV n-2 | TLV n-1 |  TLV n  || | in       | |
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    |             |          | |      |        | +---------+--   --+---------+---------+---------+| | Interest | |
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    |             |          | +------+--------+--------------------------------------------------+ +----------+ |
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    +-------------+----------+-----------------------------------------------------------------------------------+
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                                                 \                                    /\        /
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                                                  ----------------  ------------------  ---  ---
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                                                                  \/                       \/
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                                                       Signed portion of Interest       Signature
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A command interest must prevent replay attack, therefore a command interest may encode additional components before the SignatureInfo component.
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Depending on specific communication models, the command interest may introduce different name components.
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We introduce one of the simplest solutions, **stop-and-wait**:
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## stop-and-wait
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A stop-and-wait command interest introduces two additional components:
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* ``<timestamp>``
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* ``<nonce>``
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For example, for ``/command/interest/name`` name, CommandInterest will be defined as:
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     /command/interest/name/<timestamp>/<random-value>/<SignatureInfo>/<SignatureValue>
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                           \                         /\                               /
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                            ------------  -----------  ---------------  --------------
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                                        \/                            \/
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                           Command Interest Extension  Components of Signed Interest
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### Timestamp component (n-3 *th*)
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The value of the n-3 *th* component is the interest's timestamp (in terms of millisecond offset from UTC 1970-01-01 00:00:00) encoded as [nonNegativeInteger](http://named-data.net/doc/ndn-tlv/tlv.html#non-negative-integer-encoding). 
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The timestamp may be used to protect against replay attack.
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### Nonce component (n-2 *th*)
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The value of the n-2 *th* component is random value (encoded as [nonNegativeInteger](http://named-data.net/doc/ndn-tlv/tlv.html#non-negative-integer-encoding)) that adds additional assurances that the interest will be unique.
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### Command Interest Processing
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**Note that this verification process require signed interest to be received in order.  Applications adopting this process may want to take "stop-and-wait" strategy.**
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On receiving an Interest, the producer, after verifying the packet signature, should treat the Interest as invalid in the following three cases:
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- one of the two components above (Timestamp and Nonce) is missing or cannot be parsed correctly;
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- the timestamp is **equal or earlier** than the timestamp of the received one has been received before.
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Note that in order to detect this situation, the recipient needs to maintain a *latest timestamp* state for each trusted public key
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(**Since public key cryptography is used, sharing private keys is not recommended. If private key sharing is inevitable, it is the key owner's responsibility to keep clock synchronized**).
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For each trusted public key, the state is initialized as the timestamp of the first valid Interest signed by the key.
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Since then, the state will be updated every time when the recipient receives a valid signed Interest.
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Note that for the first Interest, the state is not available. 
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To handle this special situation, the recipient should check the Interest's timestamp against a grace interval (e.g., 120 seconds) [current\_timestamp - interval/2, current\_timestamp + interval/2].
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The first interest is invalid if its timestamp is outside of the interval.