Side_1_360

does not know its COA. In such cases, it may

distribute its address to the correspondent node

for more efficient routing.

For IPv6 the routing header option can be

utilised in combination with mobile IP to imple-

ment more efficient routing. When the mobile

node receives a new COA, the address can be

distributed to its HA using the destination header

option. Some essential changes in IPv6 are (see

[Pain01]):

• Route optimisation: the correspondent node
  can send packets to the mobile node without
  passing through the home agent (avoiding tri-
  angular routing).


• Filtering: allowing the mobile node to use its
  care-of-address in the source address, any fil-
  tering in intermediate routers may be passed
  more easily (compared to when a “foreign”
  address would be used). The home address
  would be carried in a destination header
  option.


• Foreign Agents not needed: by using IPv6 fea-
  tures like neighbour discovery and address
  autoconfiguration, the FAs are eliminated.


• Security: IPv6 would use IPSec for security
  requirements.


• IPv6 routing headers: introducing the routing
  header option, IP tunnelling could be obsolete,
  reducing the overhead.

Mobile IP originally primarily addressed

“slowly” moving entities, which is likely to

be combined with other features for so-called

micro-mobility. Some of these are surveyed in

[Pain01].

4.2 TCP and Wireless

A transport protocol like TCP should in princi-

ple be independent of the underlying layers.

However, considering the congestion control

algorithm used in TCP it turns out to be sensitive

to the characteristics of those layers. For in-

stance, when a segment is lost TCP assumes this

is due to network congestion and reduces its rate

of transmission. When a system introducing high

bit error rate is used, a segment could very well

be “lost” as seen from the TCP layer although all

bits have been received (albeit some bits incor-

rectly). In such a case, a proper behaviour of the

TCP layer may not be to reduce its rate, but

rather to keep, or even increase its rate. For wire-

less systems, one suggested solution is to divide

the TCP connection into two parts, each of the

parts having separate characteristics, see Figure

14. One of the parts traverses the wired part
    where lost TCP segments commonly imply con-
    gestion asking for the regular behaviour of TCP.

The other part may ask for a different behaviour,

implying that the TCP functions should be

changed. In this case the base station has to ter-

minate both TCP connections, meaning that end-

to-end connection and acknowledgements are

not present.

Another suggestion is not to split the TCP con-

nection, but to introduce an agent into the base

station. This agent will examine the TCP seg-

ments transmitted on the air interface and

retransmit the segment if an acknowledgement

has not been received within a short delay. This

is to “hide” those losses to the original sender,

which after a relatively longer delay will retrans-

mit the packets. In case there is a high bit error

ratio on the air interface, the retransmission

timer in the original sender may expire, after all

resulting in little gain. This solution could be

further extended by introducing selective re-

transmissions, suppression of acknowledgement

duplicates and so forth.

Another issue is to deal with handovers. Two

factors are that the handovers may take some

time, and that the effective throughput may be

quite different on the interfaces before and after

the handover. An example of the latter is hand-

over from a LAN to a GSM network, e.g. for an

ongoing file transfer. Hence, combining wireless

and transport protocols may imply additional

challenges.

5 Multicast

Multicast, as the term says, is traffic flowing

from one to many users at the same time. For

such applications the use of unicast, implying

sending the same information in parallel to the

receivers, would in most cases be rather ineffi-

cient.

Two essential issues for multicast are:

• How is the group of receivers identified and
  maintained;


• How is the tree for distributing information
  built.

Figure 14 Dividing into
several TCP connections

wireless

terminal/

host

base

station

TCP

connection X

TCP

connection Y

possible agent

host/

terminal