TCP Traffic Control

1
Chapter 12

• TCP Traffic Control

2
Introduction

• Performance implications of TCP Flow and Error
  Control
• Performance implications of TCP Congestion
  Control
• Performance of TCP/IP over ATM

3
TCP Flow and Error Control

• Uses a form of sliding window (GBN)
• Differs from mechanism used in LLC, HDLC, X.25,
  and others
• Decouples acknowledgement of received data units
  from granting permission to send more
• TCPs flow control is known as a credit
  allocation scheme
• Each transmitted octet is considered to have a
  sequence number
• Each acknowledgement can grant permission to send
  a specific amount of additional data
• TCP Window Size lt Min (CongWin, RcvWin)

4
TCP Header Fields for Flow Control

• Sequence number (SN) of first octet in data
  segment
• Acknowledgement number (AN) of next expected
  octet
• Window size (W) in octets
• Acknowledgement contains AN i, W j
• Octets through SN i - 1 acknowledged
• Permission is granted to send W j more octets,
• i.e., octets i through i j - 1

5
TCP Credit Allocation Mechanism
Note trailing edge advances each time A sends
data, leading edge advances only when B grants
additional credit.
6
Credit Allocation is Flexible

• Suppose last message B issued was AN i, W
  j
• To increase credit to k (k gt j) when no new data,
  B issues AN i, W k
• To acknowledge segment containing m octets (m lt
  j) without granting additional credit, B issues
  AN i m, W j - m

7
Flow Control Perspectives
8
Credit Policy

• Receiver needs a policy for how much credit to
  give sender
• Conservative approach grant credit up to limit
  of available buffer space
• May limit throughput in long-delay situations
• Optimistic approach grant credit based on
  expectation of freeing space before data arrives

9
Effect of TCP Window Size on Performance

• W TCP window size (octets)
• R Data rate (bps) at TCP source
• D Propagation delay (seconds)
• between source and destination
• After TCP source begins transmitting, it takes D
  seconds for first bits to arrive, and D seconds
  for acknowledgement to return (RTT)
• TCP source could transmit at most 2RD bits, or
  RD/4 octets

10
Maximum Normalized Throughput S

• 1 W ? RD / 4
• 4W W ? RD / 4
• RD

S
Where W window size (octets) R data rate
(bps) at TCP source D dprop (seconds) between
TCP source and destination, 2D RTT Note RD
(bits) known as rate-delay product
11
Window Scale Parameter (Optional header item)
W RD/4
W 220 - 1
W 216 - 1
RD
12
Complicating Factors

• Multiple TCP connections are multiplexed over
  same network interface, reducing data rate, R,
  per connection (?S?)
• For multi-hop connections, D is the sum of delays
  across each network plus delays at each router,
  increasing D (?S?)
• If source data rate R at source exceeds data rate
  on one of the hops, that hop will be a bottleneck
  (?S?)
• Lost segments are retransmitted, reducing
  throughput. Impact depends on retransmission
  policy (?S?)

13
Retransmission Strategy

• TCP relies exclusively on positive
  acknowledgements and retransmission on
  acknowledgement timeout
• There is no explicit negative acknowledgement
  (NAK-less)
• Retransmission required when
• Segment arrives damaged, as indicated by checksum
  error, causing receiver to discard segment
• Segment fails to arrive (implicit detection
  scheme)

14
TCP Timers

• A timer is associated with each segment as it is
  sent
• If a timer expires before the segment is
  acknowledged, sender must retransmit
• Key Design Issue
• value of retransmission timer
• Too small many unnecessary retransmissions,
  wasting network bandwidth
• Too large delay in handling lost segment

15
Two Strategies

• Timer should be longer than round-trip delay
  (send segment, receive ack)
• Round Trip Delay is variable
• Strategies
• Fixed timer
• Adaptive

16
Problems with Adaptive Scheme

• Peer TCP entity may accumulate acknowledgements
  and not acknowledge immediately
• For retransmitted segments, sender cant tell
  whether acknowledgement is response to original
  transmission or retransmission
• Network conditions may change suddenly
• These tend to introduce artificialities in
• timer measurements at the TCP source
• (more later)

17
Adaptive Retransmission Timer

• Average Round-Trip Time (ARTT)
• K 1
• ARTT(K 1) 1 ? RTT(i)
• K 1 i 1
• K ARTT(K)
  RTT(K 1)
• K 1
  K 1

18
RFC 793 Exponential Averaging

• Smoothed Round-Trip Time (SRTT)
• SRTT(K 1) a SRTT(K)
• (1 a) RTT(K 1)
• The older the observation, the less it is counted
  in the average.

19
Exponential Smoothing Coefficients

• 0.5
• 0.875

20
Exponential Averaging
Decreasing function
Increasing function
21
RFC 793 Retransmission Timeout

• RTO(K 1)
• Min(UB, Max(LB, ß SRTT(K 1)))
• UB, LB prechosen fixed upper and lower bounds
• Example values for a, ß
• 0.8 lt a lt 0.9 1.3 lt ß lt 2.0

22
TCP Implementation Policy Options (per RFC
793/1122)

• Send free to transmit when convenient
• Deliver free to deliver to application when
  convenient
• Accept how data are accepted
• In-order (out of order data is discarded)
• In-window (accept and buffer any data in window)
• Retransmit how to handle queued, but not yet
  acknowledged, data
• First-only (timer per queue, retransmit segment
  FIFO)
• Batch (timer per queue, retransmit whole queue)
• Individual (timer per segment, retransmit by
  segment)
• Acknowledge
• immediate (send immediate ack for each good
  segment)
• cumulative (timed wait, and piggyback cumulative
  ack)

Performance implications?
23
TCP Congestion Control

• Dynamic routing can alleviate congestion by
  spreading load more evenly
• But, only effective for unbalanced loads and
  brief surges in traffic
• Congestion can only be effectively controlled by
  limiting total amount of data entering the
  network
• ICMP Source Quench message is crude and not
  effective
• RSVP may help, but not widely implemented

24
TCP Congestion Control is Difficult

• IP is connectionless and stateless, with no
  provision for detecting or controlling congestion
• RFC 3168 adds ECN to IP, but not widely deployed
  yet
• TCP only provides end-to-end flow control
• No cooperative, distributed algorithm to bind
  together various TCP entities

25
TCP Flow and Congestion Control

• The rate at which a TCP entity can transmit is
  determined by rate of incoming ACKs to previous
  segments with new credit (TCP self-clocking)
• Rate of ACK arrival is determined by the
  round-trip path between source and destination
• Bottleneck may be destination or internet
• Sender cannot tell which
• Only the internet bottleneck can be due to
  congestion

26
TCP Segment Pacing Self-Clocking
Congestion Control (bottleneck in the network)
Flow Control ( bottleneck at the receiver)
27
TCP Flow and Congestion Control Potential
Bottlenecks
Physical bottlenecks physical capacity
constraints Logical bottlenecks queuing effects
due to load
28
TCP Congestion Control Measures
Note TCP Tahoe and TCP Reno from Berkeley Unix
TCP implementations
29
Retransmission Timer Management

• Three Techniques to calculate retransmission time
  out (RTO) value
• RTT Variance Estimation
• Exponential RTO Backoff
• Karns Algorithm

30
RTT Variance Estimation(Jacobsons Algorithm)

• 3 sources of high variance in RTT
• If data rate is relatively low, then transmission
  delay will be relatively large, with larger
  variance due to variance in packet size
• Load may change abruptly due to other sources
• Peer may not acknowledge segments immediately

31
Jacobsons Algorithm

• SRTT(K 1) (1 g) SRTT(K) g RTT(K 1)
• SERR(K 1) RTT(K 1) SRTT(K)
• SDEV(K 1) (1 h) SDEV(K) h SERR(K
  1)
• RTO(K 1) SRTT(K 1) f SDEV(K 1)
• g 0.125 (?)
• h 0.25 (?)
• f 2 or f 4
• (most current implementations use f 4)

32
Jacobsons RTO Calculations
Decreasing function
Increasing function
33
Two Other Factors

• Jacobsons algorithm can significantly improve
  TCP performance, but
• What RTO should we use for retransmitted
  segments?
• ANSWER exponential RTO backoff algorithm
• Which round-trip samples should we use as input
  to Jacobsons algorithm?
• ANSWER Karns algorithm

34
Exponential RTO Backoff

• Increase RTO each time the same segment is
  retransmitted backoff process
• Multiply RTO by constant
• RTO q RTO
• q 2 is called binary exponential backoff (like
  Ethernet CSMA/CD)

35
Which Round-trip Samples?

• If an ack is received for a retransmitted
  segment, there are 2 possibilities
• Ack is for first transmission
• Ack is for second transmission
• TCP source cannot distinguish between these two
  cases
• No valid way to calculate RTT
• From first transmission to ack, or
• From second transmission to ack?

36
Karns Algorithm

• Do not use measured RTT for retransmitted
  segments to update SRTT and SDEV
• Calculate exponential backoff RTO when a
  retransmission occurs
• Use backoff RTO for segments until an ack arrives
  for a segment that has not been retransmitted
• Then use Jacobsons algorithm to calculate RTO

37
Window Management

• Slow start
• Dynamic window sizing on congestion
• Fast retransmit
• Fast recovery
• ECN
• Other Mechanisms

38
Slow Start

• awnd MIN credit, cwnd
• where
• awnd allowed window in segments
• cwnd congestion window in segments (assumes MSS
  bytes per segment)
• credit amount of unused credit granted in most
  recent ack (rcvwindow)
• cwnd 1 for a new connection and increased by 1
  (except during slow start) for each ack received,
  up to a maximum

39
Effect of TCP Slow Start
40
Dynamic Window Sizing on Congestion

• A lost segment indicates congestion
• Prudent (conservative) to reset cwnd to 1 and
  begin slow start process
• May not be conservative enough easy to
  drive a network into saturation but hard for the
  net to recover (Jacobson)
• Instead, use slow start with linear growth in
  cwnd after reaching a threshold value

41
Slow Start and Congestion Avoidance
42
Illustration of Slow Start and Congestion
Avoidance
43
Fast Retransmit (TCP Tahoe)

• RTO is generally noticeably longer than actual
  RTT
• If a segment is lost, TCP may be slow to
  retransmit
• TCP rule if a segment is received out of order,
  an ack must be issued immediately for the last
  in-order segment
• Tahoe/Reno Fast Retransmit rule if 4 acks
  received for same segment (I.e. 3 duplicate
  acks), highly likely it was lost, so retransmit
  immediately, rather than waiting for timeout

44
Fast Retransmit
Triple duplicate ACK
45
Fast Recovery (TCP Reno)

• When TCP retransmits a segment using Fast
  Retransmit, a segment was assumed lost
• Congestion avoidance measures are appropriate at
  this point
• E.g., slow-start/congestion avoidance procedure
• This may be unnecessarily conservative since
  multiple ACKs indicate segments are actually
  getting through
• Fast Recovery retransmit lost segment, cut
  threshold in half, set congestion window to
  threshold 3, proceed with linear increase of
  cwnd
• This avoids initial slow-start

46
Fast Recovery Example
Tahoe Slow Start
47
Explicit Congestion Notification in TCP/IP RFC
3168

• ECN capable router sets congestion bits in the IP
  header of packets to indicate congestion
• TCP receiver sets bits in TCP ACK header to
  return congestion indication to peer (sender)
• TCP senders respond to congestion indication as
  if a packet loss had occurred

IPv4 TOS field, IPv6 Traffic Class field
48
Explicit Congestion Notification in TCP/IP RFC
3168

• 0 0 Not ECN Capable Transport
• 0 1 ECT (1)
• 1 0 ECT (0)
• 1 1 Congestion Experienced

0 0 Set-up not ECN Capable response 0
1 Receiver ECN-Echo CE packet received 1
0 Sender Congestion Window Reduced
acknowledgement 1 1 Set-up with SYN to indicate
ECN capability
49
TCP/IP ECN Protocol
TCP Sender
TCP Receiver
Hosts negotiate ECN capability during TCP
connection setup
SYN ECE CWR
SYN ACK ECE
50
TCP/IP ECN Some Other Considerations

• Sender sets CWR only on first data packet after
  ECE
• CWR also set for window reduction for any other
  reason
• ECT must not be set in retransmitted packets
• Receiver continues to send ECE in all ACKs until
  CWR received
• Delayed ACKs if any data packet has CE set, send
  ECE
• Fragmentation if any fragment has CE set, send
  ECE

51
Some Other Mechanisms for TCP Congestion Control

• Limited Transmit for small congestion windows
  triggers fast retransmit for lt 3 dup ACKs
• Appropriate Byte Counting (ABC) congestion
  window modified based number of bytes
  acknowledged by each ACK, rather than by the
  number of ACKs that arrive
• Selective Acknowledgement defines use of TCP
  selective acknowledgement option

52
Performance of TCP over ATM

• How best to manage TCPs segment size, window
  management and congestion control mechanisms
• at the same time as ATMs quality of service and
  traffic control policies
• TCP may operate end-to-end over one ATM network,
  or there may be multiple ATM LANs or WANs with
  non-ATM networks

53
TCP/IP over AAL5/ATM
54
Performance of TCP over UBR

• Buffer capacity at ATM switches is a critical
  parameter in assessing TCP throughput performance
  (why?)
• Insufficient buffer capacity results in lost TCP
  segments and retransmissions
• No segments lost if each ATM switch has buffer
  capacity equal to or greater than the sum of the
  rcvwindows for all TCP connections through the
  switch (practical?)

55
Effect of Switch Buffer Size (example - Romanow
Floyd)

• Data rate of 141 Mbps
• End-to-end propagation delay of 6 µs
• IP packet sizes of 512 9180 octets
• TCP window sizes from 8 Kbytes to 64 Kbytes
• ATM switch buffer size per port from 256 8000
  cells
• One-to-one mapping of TCP connections to ATM
  virtual circuits
• TCP sources have infinite supply of data ready

56
Performance of TCP over UBR
(UBR)
57
Observations

• If a single cell is dropped, other cells in the
  same IP datagram are unusable, yet ATM network
  forwards these useless cells to destination
• Smaller buffer increases probability of dropped
  cells
• Larger segment size increases number of useless
  cells transmitted if a single cell dropped

58
Partial Packet and Early Packet Discard

• Reduce the transmission of useless cells
• Work on a per-virtual-channel basis
• Partial Packet Discard
• If a cell is dropped, then drop all subsequent
  cells in that segment (i.e., up to and including
  the first cell with SDU type bit set to one)
• Early Packet Discard
• When a switch buffer reaches a threshold level,
  preemptively discard all cells in a segment

59
Performance of TCP over UBR
(UBR)
60
ATM Switch Buffer Layout Selective Drop and
Fair Buffer Allocation
61
EPD Fairness - Selective Drop

• Ideally, N/V cells buffered for each of the V
  virtual channels
• Weight ratio, W(i) N(i) N(i) V
• N/V
  N
• Selective Drop
• If N gt R and W(i) gt Z
• then drop next new packet on VC(i)
• Z is a parameter to be chosen (studies show
  optimal Z slightly lt 1)

62
Fair Buffer Allocation

• More aggressive dropping of packets as congestion
  increases
• Drop new packet when
• N gt R and W(i) gt Z B R
• N - R

Note that the larger the portion of the safety
zone (B-R) that is occupied, the smaller the
number with which W(i) is compared.
63
TCP over ABR

• Good performance of TCP over UBR can be achieved
  with minor adjustments to switch mechanisms
  (i.e., PPD/EPD)
• This reduces the incentive to use the more
  complex and more expensive ABR service
• Performance and fairness of ABR is quite
  sensitive to some ABR parameter settings
• Overall, ABR does not provide significant
  performance over simpler and less expensive
  UBR-EPD or UBR-EPD-FBA