Networks and Protocols
International University Bremen
Dr. Jürgen Schönwälder
Course: 320351-A
Date: 2003-11-04
Deadline: 2003-11-18
Solution #2
Solution 2.1: IPv6 auto-configuration
(10+5+5=20 points)
1. The IPv6 packet can be decoded as follows:
6000
0038
fe80
02e0
ff02
0000
8600
4000
0000
0000
0304
40e0
0027
0009
0000
2001
0000
0101
00e0
0000
3aff
0000
81ff
0000
0000
dc7f
001e
0000
0000
0000
fe25
0000
0000
8d00
3a80
0000
0638 0709
0000 0000
00e0
8125 1043
Version = 6, Traffic Class = 0, Flow Label = 0
Payload Length = 56, Next Header = 58 (ICMPv6), Hop Limit = 255
0000 Source Address = fe80::02e0:81ff:fe25:1043
1043
0000 Destination Address = ff02::1
0001
Type 134 (Router Advertisement), Code = 0, Checksum = 0xdc7f
Cur Hop Limit = 64, Router Lifetime = 30 seconds
Reachable Time = 0 milliseconds
Retrans Timer = 0 milliseconds
Option Type = 3 (Prefix Information), Length = 32 bytes
Prefix Length = 64, on-link, address-configuration
Valid Lifetime = 2592000 seconds (30 days)
Preferred Lifetime = 604800 seconds (7 days)
Reserved
0001 Prefix = 2001:638:709:1::
0000
Option Type = 1 (Source Link-layer Address), Length = 8 bytes
IEEE 802 MAC Address 00:e0:81:25:10:43
2. The MAC address 00:E0:00:C1:F2:AD is algorithmically transformed into the EUI-64 address
02:E0:00:FF:FE:C1:F2:AD. This immediately leads to the configuration of the link-local IPv6
address fe80::2e0:ff:fec1:f2ad/64. After receiving the ICMPv6 Router Advertisement with
the prefix 2001:638:709:1::/64, the global IPv6 address 2001:638:709:1:2e0:ff:fec1:f2ad/64 is
configured.
3. Routers are required to decrement the Hop Limit field. To ensure that neighbor discovery
message come from a link local source, the Hop Limit field must contain 255. An attacker
who injects neighbor discovery message for a remote point in the network will not be able
to generate a message with a Hop Limit field value of 255. However, he might be able to
generate a message with a Hop Limit field value of 1 by carefully adjusting the Hop Limit
field before sending the message.
Solution 2.2: transmission control protocol
(5+5+10=20 points)
1. If an acknowledgement is received before the associated retransmission timer expires, the
estimation of the round-trip time is updated using the formula RT T = α · RT T + (1 − α)M .
Inserting the values for α and M , we get (with RT T0 = 30):
RT T1 =
7
1
7
1
· RT T0 + 26 ms = · 30 ms + 26 ms = 29.5 ms
8
8
8
8
7
1
7
1
· RT T1 + 32 ms = · 29.5 ms + 32 ms = 29.8125 ms
8
8
8
8
7
1
7
1
RT T3 = · RT T2 + 24 ms = · 29.8125 ms + 24 ms = 29.0859375 ms
8
8
8
8
RT T2 =
2. The initial window size IW is given by the following formula:
IW = min(4 · SM SS, max(2 · SM SS, 4380)) bytes
With SM SS = 2048 bytes, we get:
IW = min(8192, max(4096, 4380)) bytes = 4380 bytes
During slow start, the congestion window is increased by at most SMSS bytes for every
acknowledgement received. We assume that there is no congestion and all acknowledgements
arrive well before the retransmission timer expires. This leads to the following behavior:
step
1
2
2
4
cwnd
4380 bytes
8476 bytes
16668 bytes
33052 bytes
description
two segments are sent and acknowledged
four segments are sent and acknowledged
eight segments are sent and acknowledged
twelve segments are sent due to the receive window
It therefore takes three round-trip times or 30 ms until the full receive window of 24 K can
be used.
3. Running TCP over TCP is not a good idea. The main reason is that the retransmission
strategy used by the lower-layer TCP will already ensure that data is reliably transmitted.
The upper-layer TCP engine is however not aware of this and will try to adapt timers and
request retransmissions. The problem now is that the timers in the upper-layer TCP engine
will be confused by the behaviour of the lower-layer TCP engine when segments are lost and
request retransmissions that queue up and can stall the lower layer TCP engine.
Solution 2.3: simple chat protocol
(30+30=60 points)
See the programs chat and chatd which are available from the course web page.