Congestion and ECN - Internetwork Programming | ECE 4110, Study notes of Electrical and Electronics Engineering

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ECE 4110

Congestion & ECN

Stevens, Chapter 21.1 – 21.

RTT estimate

• RTT

new

= (a)RTT

old

+ (1-a)RTT

pkt

• Enhanced scheme, Van Jacobson, 1988• Uses “enhanced measurement”

– Accounts for

average

plus

variance

• RTT

pkt

often measured at 500ms precision

Slow Start

• Recall: Slow Start

– Uses congestion window (CWND)

• Measured in bytes, • Incremented in segment size (MSS)

– CWND initialized to one MSS

• Grows by MSS for each packet that is received and

acknowledged before RTO

• Exponential growth

– Attempting to approximate:

• CWND = Delay x Bandwidth

Indication of Lost packet

CONGESTION

• Note that not every successfully sent packet

is ack’d^ – If 4 packets sent (ie, CWND >= 4 MSS), and

they are all received, then the receiver mayacknowledge only the last packet.

• Therefore, packet is lost if:

– 1) RTO timeout occurs (this could be a lost ack

also, but is treated as lost packet)

– 2) A duplicate ack (previously sent packet) is

received

Fast Retransmit & Fast Recovery

-^

More modifications (proposed 1990, VJ), “duplicate acks”^ –

Indicates a packet was lost but later packets arrived (CWND >>MSS) ,OR packets were reordered along the way

-^

Receiver must “ack” immediately for incorrect packet (cannot delay)

-^

If three or more “duplicate” acks received:^ –

“Mild” congestion, one packet was lost, others flowing …

-^

Don’t overreact:

• Set SSTHRESH = ½ CWND • Retransmit the missing packet • Set CWND = SSTHRESH + 3 segments
  • Add one segment to CWND for any more duplicate acks
    • On next non-duplicate ack, set CWND = SSTHRESH from above
      • Continue with normal CWND>SSTHRESH congestion avoidance

Network Implications: Fairness

• In Internet backbone:,

– congestion occurs when a large number of hosts

exceed the link capacity.

• Verses finding “modem speed” in a single user

perspective

– Each active sender dynamically seeks to

achieve the same level of packet loss

– More loss -> slower throughput – Less loss -> faster throughput

Explicit Congestion Notification

•^

RFC 3168 (Sept 2001):

http://www.ietf.org/rfc/rfc3168.txt

• Instead of dropping a packet, set a “congestion” bit in

the packet header

• 1. Negotiate ECN capable sender/receiver
  • Use bit pair in

IP

header (NOT TCP header)

• Set bit pair to 10 or 01 to indication ECN capable – Leave at 00 for not ecn capable
  • 2. Queues set congestion bit pair to 11 when consistently near

full

• This is a notification to the receiver that the sender-> receiver

link was full

• 3. Receiver sets new bit from RESERVED field in TCP header

(receiver tells sender) called ECN-Echo (ECE)

Sender reacts to ECE bit in ack packet as if a packet was lost (congestion avoidance),

without resending

• 5. Sender replies with TCP RESERVED field CWR bit to let

receiver know that it is responding to congestion

Comments:

•^

Many things can change:^ – Sender doesn’t have to “retransmit” identical packets

(ie, it may group data into larger packet)

• TCP “Sender” and “Receiver” are intermediate

processing. TCP does not generate or use data.Applications can generate/use data in sporadic ways.TCP must accommodate …

• TCP does not normally “monitor” the connection, ie,

and idle connection sends no packets.