Download Factors that influence TCP performance

TCP Functions


TCP is a connection oriented protocol
Primary functions

TCP sets up and maintains end-to-end connection between two hosts



Ensures reliable delivery of packets





Selective Repeat ARQ protocol
Positive ACK of packets
Sequence numbers
Error detection
Enforces flow control


Setup and teardown of connection are distinct phases
A connection is defined by the quadruplet of IP addresses and port numbers of both
hosts
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)
 Time out (adjust threMGold, return to slow start)
ECE 695 Sp 2006
Jim Catt
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
ECE 695 Sp 2006
Jim Catt
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 don’t 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 MN’s 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.
ECE 695 Sp 2006
Jim Catt
Mobile TCP Strategies

Utilize the link layer to detect and correct errors

Hides errors from TCP, avoids triggering inappropriate responses


Split TCP connections into distinct connections

Treats wired and wireless connections differently to accommodate
different characteristics


Snoop TCP, TCP-Unaware Link Layer
Indirect TCP, Mobile TCP
Re-engineer TCP to accommodate wireless

Add features or explicitly modify behavior of protocol in response to
certain events.



Modify protocol to improve efficiency


Handle loss events that are not due to congestion differently
Anticipate loss event due to handoffs, proactively manage these
Improves bandwidth utilization and lowers probability of packet loss
Explicit Loss Notice, WTCP, TCP SACK, T/TCP
ECE 695 Sp 2006
Jim Catt
Link Layer Strategies

Snoop TCP
 Buffer
data as close as possible to MN to minimize retransmit
 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
ECE 695 Sp 2006
Jim Catt
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
ECE 695 Sp 2006
Jim Catt
Split Connection Strategies

Indirect TCP (ITCP)

Splits single TCP connection into two connections





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



MS-to-BS (AP) (wireless)
BS-to–CN (wired)
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
ECE 695 Sp 2006
Jim Catt
Split Connection Strategies

Mobile TCP (M-TCP)

TCP connection is split at a mobile-gateway (MG)


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






Part of the fixed network. May be an AP or controller for multiple APs
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
ECE 695 Sp 2006
Jim Catt
Mobile-TCP Protocol Stack
Source: Z. Haas., P. Agrawal, “ Mobile-TCP: An Asymmetric Transport Protocol Design for
Mobile Systems”, IEEE, 2000.
ECE 695 Sp 2006
Jim Catt
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 shouldn’t 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 isn’t 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.
ECE 695 Sp 2006
Jim Catt
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)
ECE 695 Sp 2006
Jim Catt
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.

ECE 695 Sp 2006
Jim Catt
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 setup, tear-down and data transfer into a single
transaction


Combine reliability of TCP with the simplicity and speed of UDP
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

Doesn’t solve the problem of maintaining continuity between
handoffs in the mobile case, i.e., doesn’t address packet loss in
such cases.
ECE 695 Sp 2006
Jim Catt
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
ECE 695 Sp 2006
Jim Catt