M-TCP : TCP for Mobile Cellular Networks

1
M-TCP TCP for Mobile Cellular Networks

• Kevin Brown and Suresh Singh
• Department of Computer Science
• Univ. of South Carolina
• ACM Computer Communications Review, 1997
• 2005.07.29
• Hyun-Jin Kim

2
Introduction TCP in Mobile Environment

• Traditional TCP
• TCP is designed to work on wired networks
• Negligible medium loss
• Buffer overflow at routers leads network
  congestion
• TCP under wireless mobile networks
• High BER and frequent disconnections lead packet
  losses
• But, TCP simply interprets them as a indication
  of congestion
• Significant degradation of end-to-end performance
• Approaches for improving TCP performance
• Snoop TCP 1
• I-TCP2 MTCP3
• M-TCP

3
Former Solutions SNOOP TCP

• Snoop module resides at the BS (Base Station)
• Inspects TCP header of data packets and ACK
  packets
• Keeps track of packets in both direction
• Local retransmission between BS and MH (Mobile
  Host)
• Pros
• Preserves end-to-end TCP semantics
• Changes are restricted to BS and optionally to MH
• Cons
• Under long and frequent disconnections, sender
  times out
• Frequent handoffs snoop module needs to build
  its cache up

4
Former Solutions I-TCP MTCP

• Split connection protocols
• FH to BS, BS to MH
• Wireless connection can even use another
  transport protocol that suits wireless medium
• Pros
• When a handoff occurs, the old BS can deliver the
  packets of its buffer to the new BS
• Sender does not concern about wireless
  environment
• Cons
• Breaks end-to-end TCP semantics
• BS can be a bottleneck

BS
FH
MH
Wireless TCP
Standard TCP
FH Socket?? MH Socket
5
M-TCP - Overview

• TCP connection is split in two at the SH
• Maintains end-to-end TCP semantics
• Persist mode keeps timer of sender
• SH (Supervisor Host)
• Maintains several MSS (Mobile Support Station)
  reduces handoff overheads
• Serves function of gateway
• Bandwidth manager, local recovery

6
M-TCP The SH-TCP client

• Goal keeps the FHs congestion window size
• FH uses unmodified TCP to send data to the SH
• SH-TCP client
• Passes packets from the FH on to M-TCP
• Does not ACK those packets until the MH does
• So, end-to-end TCP semantics are maintained
• Sends ACK to the sender except ACK for one last
  byte
• When the MH is disconnected, sends ACK for the
  last byte with a window size set to 0
  persist mode
• When a MH regains its connection, it sends a
  greeting packet
• This packet allows the sender to leave persist
  mode

7
M-TCP M-TCP between the SH and MH

• Goal fast local recovery from disconnection
  events
• Has responds to notifications of wireless link
  connectivity
• M-TCP at the MH is notified the connection is
  lost
• Freezes all M-TCP timer
• Connection is regained
• Unfreezes all M-TCP timers
• MH sends a specially marked ACK to M-TCP at the
  SH which contains the highest received sequence
  number
• How to determine the connection is lost?
• Assumes the SH assigns fixed bandwidth
• No ACK from the MH within Timeout of M-TCP (See
  next slide)

8
M-TCP How to estimate RTO ?

• Therefore, the SH can estimate RTO between itself
  and the MH
• The SH can determine the connection is lost by
  using estimated RTO

(1) FTCPRTO (24) STCPRTO (3)
SM-TCPRTO Therefore (1) (24)(3)
9
M-TCP Other Issues

• Compressed M-TCP
• Packets are compressed at the SH and decompressed
  at the MH
• When a handoff occurs
• Freezes connection state
• Some of the state about the connection is passed
  to the new SH
• Removes the contents of the socket buffers at the
  old SH
• Unfreezes connection state
• Connection setup
• Two different operations for setting up
• FH ?? SH and SH ?? MH
• Transparent SH
• FH ?? MH
• SH automatically creates sockets for the FA and
  the MH

10
M-TCP - Normal Transmission (1/2)
(2,1) buffered
SH
2 1
2 1
1
1 2
cwnd2
cwnd3
SH-TCP
M-TCP
1 2
2
This is for freezing senders window.
11
M-TCP - Normal Transmission (2/2)
(4,3) buffered
SH
4 3
4 3
3 2
3 4
cwnd3
cwnd5
M-TCP
SH-TCP
3 4
4
2
This is for freezing senders window.
12
M-TCP - Disconnection
(8,7,6,5) buffered
SH
8 7 6 5
4
cwnd5
cwnd6
Freezing
Freezing
M-TCP
SH-TCP
Freezing
4
Notify disconnection
13
M-TCP - Recovery
(8,7,6,5) buffered
SH
5 6 7 8
4
16 15 14 13 12 11 10 9
Notify reconn.
5 6 7 8
5 6 7
cwnd6
cwnd9
M-TCP
SH-TCP
8
5 6 7 8
14
M-TCP - Performance Evaluation

• Experimental set up
• All nodes are Pentium PCs
• Wireless link is emulated at the SH
• 32Kbps downlink, 8Kbps uplink
• Disconnection length 0.5 4.5 sec
• Cell latency mean 5 sec (12 disconnection
  events in 5 sec)

FH1 Distance Sender
SH
MH
Emulated 32kbps Link
14 Hops
FH2 Close Sender
4 Hops
15
M-TCP - Performance Evaluation

• M-TCP vs TCP performance

16
M-TCP - Performance Evaluation

• Compressed M-TCP vs normal M-TCP

17
M-TCP - Performance Evaluation

• M-TCP processing time

18
Conclusion

• M-TCP
• Solution to improve TCP in mobile networks
• Splits TCP connection, but it makes an effort to
  maintain end-to-end TCP semantics
• Persist mode
• Still exists problems
• When the MH sends cumulative ACK
• When the FH finishes to send data
• High processing at SH
• Does M-TCP really maintain end-to-end TCP
  semantics?

19
References

• 1 H. Balakrishnan, S.Seshan, and Randy Katz,
  Improving Reliable Transport and Handoff
  Performance in Cellular Wireless Networks,
  Wireless Networks, Vol 1 No.4 December 1995
• 2 A.Bakre and B.R.Badrinath, I-TCP Indirect
  TCP for Mobile Hosts, IC on Distributed
  Computing Systems 1995
• 3 R. Yavatkar and N. Bhagawat, Improving
  End-to-End Performance of TCP over Mobile
  Internetworks, IEEE Workshop on Mobile Computing
  Systems and Applications, 1994