receiver
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...
The correct operation of ECN requires the cooperation of the receiver
to return Congestion Experienced signals to the sender ...
... sender, but the
protocol lacks a mechanism to enforce this cooperation. This raises
the possibility that an unscrupulous or poorly implemented receiver
could always clear ECN-Echo ...
... congestion signals
to the sender. This would give the receiver a performance advantage
at the expense of competing connections ...
... sender to verify the correct behavior of
the ECN receiver and that there is no other interference that
conceals marked (or dropped) packets in the signaling path. The ECN ...
... nonce:
- catches a misbehaving receiver with a high probability, and never
implicates an innocent receiver.
...
... - catches a misbehaving receiver with a high probability, and never
implicates an innocent receiver.
- does not change other aspects of ECN ...
... ECN, nor does it reduce the
benefits of ECN for behaving receivers.
- is cheap in both per-packet ...
... nonce. We present an
overview followed by detailed behavior at senders and receivers.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
...
... The ECN-nonce adds to this protocol, and enables the receiver to
demonstrate to the sender that segments ...
... the correct nonce sum implies receipt of only unmarked packets. Not
only are receivers prevented from concealing marked packets, middle-
boxes along the network path cannot unmark a packet without
...
... The sender can verify the nonce sum returned by the receiver to
ensure that congestion indications in the form of marked (or dropped)
...
... bit long, senders have a 50-50 chance of catching a lying receiver
whenever an acknowledgement conceals a mark. Because each
acknowledgement is an independent trial, cheaters will be caught
...
... order to conceal a marked packet. The nonce sum prevents the
receiver from concealing individual marked packets by not
acknowledging them. Because the nonce and nonce ...
... After congestion has occurred and packets have been marked or lost,
resynchronization of the sender and receiver nonce sums is needed.
When packets are marked, the nonce ...
... nonce is cleared, and the sum of the
nonces at the receiver will no longer match the sum at the sender.
Once nonces ...
... sender.
Once nonces have been lost, the difference between sender and
receiver nonce sums is constant until there is further loss. This
means that it is possible to resynchronize the sender and receiver ...
... sender and
receiver nonce sums is constant until there is further loss. This
means that it is possible to resynchronize the sender and receiver
after congestion by having the sender ...
... sender set its nonce sum to that of
the receiver. Because congestion indications do not need to be
conveyed more frequently than once per round trip ...
... suspends checking while the CWR signal is being delivered and resets
its nonce sum to the receiver's when new data is acknowledged. This
has the benefit that the receiver is not explicitly involved in the
...
... nonce sum to the receiver's when new data is acknowledged. This
has the benefit that the receiver is not explicitly involved in the
re-synchronization process. The resynchronization process is shown
...
...
Figure 2: The calculation of nonce sums at the receiver when a
packet (4:8) is marked. The receiver may calculate the wrong
...
... nonce sums at the receiver when a
packet (4:8) is marked. The receiver may calculate the wrong
nonce sum when the original nonce ...
... need not match the transmitted boundaries, and information can be
retransmitted in packets with different byte boundaries. We discuss
the first issue, how a receiver sets a nonce when acknowledging part
of a segment ...
... nonce, in ECT(0) or
ECT(1), is removed. Neither the receiver nor any other party can
unmark the packet without successfully guessing the value of the
original nonce ...
... Receiver Behavior (Receive and Transmit) ...
... ECN-nonce receivers maintain the nonce sum as in-order packets arrive
and return the current nonce ...
... nonce sum as in-order packets arrive
and return the current nonce sum in each acknowledgement. Receiver
behavior is otherwise unchanged from [RFC3168]. Returning the nonce
...
... senders are allowed to
discontinue sending ECN-capable packets to receivers that do not
support the ECN-nonce ...
... In the case of marked packets, one or more nonce values may be
unknown to the receiver. In this case the missing nonce values are
ignored when calculating the sum (or equivalently a value of zero is
...
... If ECN-Echo is not set, the receiver claims to have received no
marked packets, and can therefore compute and return the correct
nonce ...
... marked packets, and can therefore compute and return the correct
nonce sum. To conceal a mark, the receiver must successfully guess
the sum of the nonces that it did not receive, because at least one
...
... sender. Because each new acknowledgement is an independent trial, a
cheating receiver is likely to be caught after a small number of
lies.
...
... If ECN-Echo is set, the receiver is sending a congestion signal and
it is not necessary to check the nonce ...
...
After recovery, it is necessary to re-synchronize the sender and
receiver nonce sums so that further acknowledgments can be checked.
When the receiver ...
... sender and
receiver nonce sums so that further acknowledgments can be checked.
When the receiver's sum is incorrect, it will remain incorrect until
further loss.
...
... sender
resets its nonce sum to that of the receiver when it receives an
acknowledgment for new data sent after the congestion window was
...
...
Figure 4: The calculation of nonce sums at the receiver when a
packet is lost, and resynchronization after loss. The nonce sum
...
... at all is optional, and may be disabled.
If the receiver has never sent a non-zero nonce sum, the sender ...
... nonce sum, the sender can
infer that the receiver does not understand the nonce, and rate limit
the connection ...
... to improve TCP performance. A misbehaving receiver might claim to
have received only original transmissions to convince the sender to
...
... bit with ECN is
recommended. Receivers that receive unmarked fragments can
reconstruct the original nonce ...
... ECN-nonce cannot protect against misbehaving receivers that conceal
marked fragments, so some protection is lost in situations where Path
MTU discovery ...
...
Selective acknowledgements allow receivers to acknowledge out of
order segments as an optimization. It is not necessary to modify the
...
... range nonce sums, because
SACKs cannot be used by a receiver to hide a congestion signal. The
nonce ...
... signaling mechanism
that improves ECN's robustness by preventing receivers from
concealing marked (or dropped) packets. The intent of this work is
to help improve the robustness of congestion control ...
... S. Savage, N. Cardwell, D. Wetherall, T. Anderson. TCP congestion control with a misbehaving receiver. SIGCOMM CCR, October 1999. ...