CISC 856: TCP/IP and Upper Layer Protocols
Document: Homework – The Eifel Detection Algorithm
Presented by: Maitreya Natu (natu@cis.udel.edu)
Due Date: Nov 18th, 2004
Q1. Describe, by adding to the timing diagram below, the sender behavior in each of the
following scenarios:
a) TCP without Eifel
b) TCP with Eifel
You have to submit 2 timing diagrams.
For each scenario indicate the following:
1. PDUs sent by the sender at points A, B and C.
2. Sender’s cwnd and ssthresh values just after the timeout, and after receiving each
ACK (A2, A3, A4).
3. In your annotated timing diagram for b, indicate the timestamp values sent with
the PDUs.
(sender and receiver timestamp values at various points are shown in the
following figure in italics. Assume that prior to sending D1, the sender had
received a timestamp of 150 from the receiver)
Sender
Receiver
ssthresh=16
cwnd=8
200
D1
D8
.
.
.
A2
A3
Time out
500
A
505
B
510
C
515
A4
300
300
300
D1
600
Q2. Describe, by adding to the timing diagram below, the sender behavior in each of the
following scenarios:
a) TCP with Eifel
b) TCP with DSACK based spurious retransmission detection
For each scenario, indicate the following:
1. PDUs sent by the sender at point A, B, C and D.
(Assume the sender always has new data available to send. Use names D5, D6,…,
for new data)
2. Sender’s cwnd and ssthresh values after timeout and after receiving each ACK.
(Assume sender remains in slow start when cwnd = ssthresh)
3. In your annotated timing diagram for a, indicate the timestamp values sent with
the PDUs.
4. In your annotated timing diagram for b, indicate the D-SACKs (if any) sent with
the ACKs.
5. Show when a spurious retransmission is detected in both a and b. Comment on
which detection policy is better in the given scenario.
Sender
200
D1
D2
D3
D4
ssthresh=16
cwnd=4
Receiver
A2
A3
A4
Time out
A5
500
A
505
B
510
C
515
D
520
300
300
300
D1
600
200
Extra Credit:
The specification of TCP round-trip time measurement requires that the echoed
timestamp should correspond to the most recent data segment that advanced the window.
Thus at point B in the timing diagram below, the receiver echoes back the sender
timestamp 100 instead of timestamp 400, because 1st PDU with data D1 and sender
timestamp 100, was the most recent PDU that advanced the window. How will the Eifel
algorithm behave in the following scenario? What will the sender do at points A and C?
Can you see a problem with Eifel algorithm? How will the Eifel algorithm recover?
Sender
ssthresh=8
cwnd=4
100
Receiver
D1, 100, 50
D2, 100, 50
D3, 100, 50
D4, 100, 50
A2, 200, 100 200
Time out
400
A
D1, 400, 50
600
A2, 600, 100
B
C
Reference:
 RFC 1323, TCP Extensions for High Performance
 RFC 2883, An Extension to the Selective Acknowledgement (SACK) Option for
TCP
 RFC 3708, Using TCP Duplicate Selective Acknowledgement (DSACKs) and
Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence
Numbers (TSNs) to Detect Spurious Retransmissions
 Pasi Sarolahti , Markku Kojo , Kimmo Raatikainen, F-RTO: an enhanced
recovery algorithm for TCP retransmission timeouts, ACM SIGCOMM
Computer Communication Review, v.33 n.2, April 2003