ECE 695 Sp 2006

1
TCP Functions

• TCP is a connection oriented protocol
• Primary functions
• TCP sets up and maintains end-to-end connection
  between two hosts
• Setup and teardown of connection are distinct
  phases
• A connection is defined by the quadruplet of IP
  addresses and port numbers of both hosts
• Ensures reliable delivery of packets
• Selective Repeat ARQ protocol
• Positive ACK of packets
• Sequence numbers
• Error detection
• Enforces flow control
• Sliding window flow control
• Receiver advertises size of window (max number of
  packets that can be sent by sender without ACK)
• Prevents buffer overflow at receiver
• Applies Congestion control
• Traditional TCP assumes losses are due to
  congestion, not errors
• 3-phase process
• Slow start (exponential increase in number of
  packets sent)
• Congestion avoidance (linear increase in number
  of packets sent)

2
Factors that influence TCP performance

• Wired environment
• Sender and receiver have static IP addresses (at
  least for the duration of the session)
• No handoffs
• While packet routes may vary, routing nodes form
  a fixed network, i.e., they do not drop into and
  out of the network and they maintain a fixed
  topology relative to one another
• facilitates the use of routing tables
• the number of potential routes, though possibly
  large, forms a fixed set
• RTT variance is lower.
• Channel conditions tend to be relatively stable
  (no fading, Doppler, multi-path)
• Packet loss is primarily due to congestion, not
  channel conditions

3
Factors that influence TCP performance

• Wireless environment
• Sender and receiver may both be mobile during the
  session, and traverse domain boundaries several
  times, requiring handoffs.
• Traditional static IP addresses dont work very
  well connections could be lost and
  re-establiMGed many times
• IP addressing is handled in mobile IP and can be
  transparent to TCP layer
• Since mobile nodes (MNs) may associate with
  different APs during a session, routing becomes
  more challenging
• core network topology may be relatively static as
  in wired case, but MNs attachment to the core
  network is dynamic
• routing tables relative to the MNs involved in
  the session grow stale more quickly
• RTT variance is higher, packet latency is higher
• Channel conditions tend to be relatively less
  stable -- fading, Doppler, multi-path and
  handoffs frequently cause packet loss
• PHY and link layers in wireless have been adapted
  to mitigate these factors, but, cannot completely
  overcome them
• Due to latency and packet loss, congestion
  control mechanisms may be triggered which cause
  needless reduction in throughput if traditional
  TCP is utilized in wireless.

4
Mobile TCP Strategies

• Utilize the link layer to detect and correct
  errors
• Hides errors from TCP, avoids triggering
  inappropriate responses
• Snoop TCP, TCP-Unaware Link Layer
• Split TCP connections into distinct connections
• Treats wired and wireless connections differently
  to accommodate different characteristics
• Indirect TCP, Mobile TCP
• Re-engineer TCP to accommodate wireless
• Add features or explicitly modify behavior of
  protocol in response to certain events.
• Handle loss events that are not due to congestion
  differently
• Anticipate loss event due to handoffs,
  proactively manage these
• Modify protocol to improve efficiency
• Improves bandwidth utilization and lowers
  probability of packet loss
• Explicit Loss Notice, WTCP, TCP SACK, T/TCP

5
Link Layer Strategies

• Snoop TCP
• Buffer data as close as possible to MN to
  minimize re-transmit
• Requires BS link layer to snoop (look into) TCP
  packets
• BS (AP) buffers packets
• Packets are removed from buffer on ACK
• Missing ACK or DUPACK causes retransmit of
  packet(s) from BS buffer
• BS does not initiate ACKs, but may initiate NACKs
  if gap in sequence number is detected

6
Link Layer Strategies

• TCP-Unaware Link Layer
• Similar to Snoop TCP, but BS link layer is
  unaffected
• Link level ACKs are used to trigger
  re-transmission of packets
• MN uses delayed DUPACKs
• Performance is sensitive to delay value

7
Split Connection Strategies

• Indirect TCP (ITCP)
• Splits single TCP connection into two connections
• MS-to-BS (AP) (wireless)
• BS-toCN (wired)
• BS-to-CN connection can used conventional wired
  TCP
• MS-to-BS connection uses customized TCP for
  wireless
• BS (AP) acts as proxy for MN
• Buffers packets destined for MN and sends ACKs to
  CN.
• Handoffs may require buffered packets to be
  forwarded to new AP
• Buffer at BS (AP) may overflow in cases of
  extended loss of connection between it and the MN

8
Split Connection Strategies

• Mobile TCP (M-TCP)
• TCP connection is split at a mobile-gateway (MG)
• Part of the fixed network. May be an AP or
  controller for multiple APs
• TCP for the wireless segment is modified
• Uses an asymmetric protocol that reduces the
  overhead at the MN
• The MN to MG segment is basically a single hop
  efficiencies can be obtained from this context
• Addressing can be simplified (use CID instead of
  full TCP addr block)
• Sequence numbers based on packets, not bytes
• No need for sliding window (no place for packets
  to circulate)
• No timers on MN side
• Congestion control is eliminated Only Flow
  control is used, based mainly on bit flags
• Because a single MG may be managing multiple APs
  (and hence, even more MNs), buffer requirements
  and complexity are increased.
• e.g., MG must maintain a state machine for each MN

9
Mobile-TCP Protocol Stack
Source Z. Haas., P. Agrawal, Mobile-TCP An
Asymmetric Transport Protocol Design for Mobile
Systems, IEEE, 2000.
10
Re-engineered TCP Strategies

• Explicit Loss Notification (ELN)
• The main idea is to distinguish between types of
  packet losses, i.e., congestion losses vs other
  types, and treat them differently
• Losses due to issues such as handoffs shouldnt
  require the same correction mechanisms as losses
  due to congestion
• Approach use MAC layer to distinguish between
  types of losses
• If MAC detects that loss isnt due to congestion,
  then the TCP layer in the MN sends an ELN
• This tells the sender that only retransmission of
  lost packets is required, NOT a reduction in the
  congestion window.

11
Re-engineered TCP Strategies

• WTCP
• Major re-engineering of TCP
• Idea add adaptive management functions that
  acknowledge the inherent differences between
  wired and wireless channels, then monitor channel
  conditions (use metrics) and adapt operations to
  current conditions
• rate-based transmission
• inter-packet separation metric
• packet loss detection mechanisms
• bandwidth estimation
• separate congestion control and reliability
  mechanisms (e.g. different sequence numbers)

12
Re-engineered TCP Strategies

• TCP SACK (Selective ACK)(1996)
• An adaptive scheme that employs the idea that
  selective retransmission is more efficient than
  non-adaptive schemes such as Go-back-N.
• Normally, receiver only ACKs latest packet
  received in order.
• This forces the sender to go-back-N packets and
  re-transmit the entire block after a timeout
  expires on the remainder of the packets.
• This is wasteful if only some of the packets in
  the block (window) were lost.
• When an out-of-order segment arrives at the
  receiver, a selective ACK is sent
• The sender only re-transmits the non-ACKed
  packets
• Transmitter maintains a metric related to number
  of packets in flight. As long as this metric is
  less than the congestion window size, the (NACK)
  segments can be re-transmitted
• Requires both sender and receiver to maintain
  additional information about packets sent and
  received.

13
Re-engineered TCP Strategies

• Transaction-Oriented TCP (1994)
• Introduced as an experimental extension to TCP
• Goal is to avoid overhead associated with setup
  and tear-down transactions
• Combine reliability of TCP with the simplicity
  and speed of UDP
• Combine setup, tear-down and data transfer into a
  single transaction
• More efficient fewer packets and less time
  required for the connection
• Originally, not envisioned as a wireless solution
  targeted to improve speed of existing wired
  network transactions
• Doesnt solve the problem of maintaining
  continuity between handoffs in the mobile case,
  i.e., doesnt address packet loss in such cases.

14
Transaction Diagrams
Traditional TCP 3-way handshake to establish a
connection. No data has transferred at this point.
T-TCP transaction that combines connection setup
with data transfer
Source http//www.linuxgazette.com/issue47/stacey
.html