Preparation for Recitation 11
Read TCP Congestion
Control with a Misbehaving Receiver by Stefan Savage et al. (reading 13 in the course packet.)
Underlying the design of TCP is an assumption that senders and
receivers will abide by TCP's congestion control algorithm. A sender
or a receiver may want to cheat and deactivate congestion control
however. In particular, honest TCP senders back off in response to
drops, reducing their transmission rates. In contrast, a cheating
sender may increase its rate and grab the capacity that the honest
senders release. This will cause more drops and thus the honest sender
will reduce their rates further, letting the cheater grab an even
bigger share of the bottleneck capacity.
A receiver also may be interested in cheating to make its FTP or
Web download go faster. It is not immediately clear that a receiver
can force an honest TCP sender to transmit more than its fair share,
if the latter abides by TCP congestion control. This paper was the
first to point out that a misbehaving receiver can actually fool an
honest sender to send faster than it should.
Note that the paper refers to packets as segments. Also note that,
in lectures, we simplified the TCP ACK scheme by saying that each data
packet has a sequence number. In reality, each data packet is
identified by the sequence of bytes it contains. For example packet
1:1460 contains the byte sequence 1 up to 1460. ACKs ask for the next
byte that the receiver expect to receive, e.g., ACK 1460, means that
the receiver has received all bytes up to byte 1460. Also, note that TCP has
more components than the two we talked about in
lectures (congestion avoidance (AIMD) and fast retransmit
(use 3 duplicate ACKs as a sign of loss)). In particular, the paper mentions the
slow-start and fast-recovery components. Check RFC 2581 for more
information on these algorithms.
After reading the paper, try to answer the following questions:
- Attacks that change TCP as described in this paper are not
common. Can you think why this is the case?
- Why it is wiser for a misbehaving receiver to use Attack 1 or 2
instead of Attack 3?