MOBILE TRANSPORT LAYER

1
MOBILE TRANSPORT LAYER

• Mobility on congestion control mechanisms
• Current proposed solutions

2
Mobility on congestion control mechanisms (I)

• TCP assumes congestion if packets are dropped.
• Typically wrong in wireless networks, here we
  often have packet loss due to transmission
  errors.
• Furthermore, mobility itself can cause packet
  loss, if e.g. a mobile node roams from one access
  point (e.g. foreign agent in Mobile IP) to
  another while there are still packets in transit
  to the wrong access point and forwarding is not
  possible.

3
Mobility on congestion control mechanisms (II)

• The performance of an unchanged TCP degrades
  severely.
• Arguments about TCP that it cannot be changed
  fundamentally due to the large base of
  installation in the fixed network, TCP for
  mobility has to remain compatible.
• The basic TCP mechanisms keep the whole Internet
  together.

4
Mobility on congestion control mechanisms (III)

• TCP Reno, the current TCP implementation, reacts
  to a wireless loss with a drastic reduction of
  the congestion window, hence, of the sender
  transmission rate.
• Effective congestion control for heterogeneous
  (wired and wireless) networks has been an active
  area of research.

5
Mobility on congestion control mechanisms (IV)

• Alternative approaches to address the provision
  of congestion control have been suggested
• End-to-end approach
• Split Connection approach (splits TCP connection
  into a fixed and wireless part)
• Link Layer approach (local error recovery)
• The best performing approach is shown to be the
  link layer one. A localized link layer solution
  is applied directly to the wireless links (e.g.,
  the Snoop protocol).

6
Mobility on congestion control mechanisms (V)

• On the other hand, End-to-end schemes can achieve
  significant gains without extensive support at
  the network layer in routers and base stations.
• Link Layer recovery is, in general, much more
  powerful than the end-to-end recovery, since it
  isolates and corrects the loss locally.

7
Current proposed solutions (I)

• End-to-End proposed solution
• TCP Selective Acknowledgement (TCP SACK)
• TCP acknowledgements are often cumulative
• ACK n acknowledges correct and in-sequence
  receipt of packets up to n
• if single packets are missing quite often a whole
  packet sequence beginning at the gap has to be
  retransmitted (go-back-n), thus wasting bandwidth

8
Current proposed solutions (II)

• Selective retransmission as one solution
• It allows for acknowledgements of single packets,
  not only acknowledgements of in-sequence packet
  streams without gaps
• sender can now retransmit only the missing
  packets
• Advantage
• much higher efficiency the sender retransmits
  only the lost packets
• Disadvantage
• more complex software in a receiver, more buffer
  needed at the receiver

9
Current proposed solutions (III)

• Split Connection proposed solution
• Indirect TCP (I-TCP)
• I-TCP splits a TCP connection into a fixed part
  and a wireless part at the base station.

wired Internet
10
Current proposed solutions (IV)

• Between the fixed computer and the access point,
  standard TCP is used.
• The access point terminates the standard TCP
  connection, acting as a proxy. This means that
  the access point is now seen as the mobile host
  for the fixed host and as the fixed host for the
  mobile host.
• Between the access point and the mobile host, a
  special TCP, adapted to wireless links, is used.
  However, even an unchanged TCP can benefit from
  the much shorter round trip time, thus starting
  retransmission much faster.

11
Current proposed solutions (V)

• The correspondent host in the fixed network does
  not notice the wireless link or the segmentation
  of the connection.
• If the correspondent host sends a packet, the
  access point acknowledges this packet. Then the
  access point tries to forward the packet to the
  mobile host. If the mobile host receives the
  packet, it acknowledges the packet. However, this
  ack is only used by the access point.

12
Current proposed solutions (VI)

• If a packet is lost on the wireless link due to
  a transmission error, the correspondent host
  would not notice it. However, the access point
  tries to retransmit this packet locally to
  maintain reliable data transport.
• Advantages
• no changes in the fixed network necessary, no
  changes for the hosts (TCP protocol) necessary,
  all current optimizations to TCP will work
• transmission errors on the wireless link do not
  propagate into the fixed network

13
Current proposed solutions (VII)

• simple to control, mobile TCP is used only for
  one hop between, e.g., a foreign agent and mobile
  host
• a very fast retransmission of packets is
  possible, the short delay on the mobile hop is
  known
• Disadvantages
• loss of the original end-to-end TCP semantic, an
  acknowledgement to a sender does not now any
  longer mean that a receiver really got a packet,
  foreign agents might crash.
• higher latency possible due to buffering of data
  within the foreign agent and forwarding to a new
  foreign agent

14
Current proposed solutions (VIII)

• Link Layer proposed solution
• Snoop protocol
• It works completely transparently and leaves the
  TCP end-to-end connection intact.
• It buffers data at the access point to perform
  fast local retransmission in case of packet loss.

15
Current proposed solutions (IX)

• The foreign agent buffers all packets with
  destination mobile host and additionally snoops
  the packet flow in both directions to recognize
  acknowledgements.

wired Internet
16
Current proposed solutions (X)

• The reason for buffering packets toward the
  mobile node is to enable the foreign agent to
  perform a local retransmission in case of packet
  loss on the wireless link.
• The foreign agent buffers every packet until it
  receives an ack from the mobile host.
• If the foreign agent does not receive an ack from
  the mobile host within a certain amount of time,
  either the packet or the ack was lost.
  Alternatively, the foreign agent could receive a
  duplicate ack also showing the loss of a packet.

17
Current proposed solutions (XI)

• Now the foreign agent retransmits the packet
  directly from the buffer, thus performing a much
  faster retransmission compared to the
  correspondent host.
• The time-out for acks can be set much shorter,
  for it reflects only the delay of one hop plus
  processing time.
• To remain transparent, the foreign agent must not
  acknowledge data to the correspondent host.
• Doing so would make the correspondent host
  believe that the mobile host had received the
  data.

18
Current proposed solutions (XII)

• This would violate the end-to-end semantic in
  case of a foreign agent failure. However, the
  foreign agent can filter the duplicate acks to
  avoid unnecessary retransmissions of data from
  the correspondent host.
• If the foreign agent crashes, the time-out of the
  correspondent host still works and triggers a
  retransmission.
• Furthermore, the foreign agent may discard
  duplicates of packets already retransmitted
  locally and acknowledged by the mobile host. This
  avoids unnecessary traffic on the wireless link.