Modeling TCP Throughput: A Simple Model and its Empirical Validation

Ross RosemarkPenn State University

Goals of Presentation

● Intro to TCP

– Particularly discuss the “Reno flavor of TCP”

– Show how TCP is innately a self tuning algorithm

● Discuss the goals of this paper

– State assumptions

– Note I do not discus in details any math.

● Discuss Analysis

– State goals of analysis

– State analysis conclusions

● Analyize Paper

– What contributions does it make

● Other these contributions significant?

What is TCP?

● According to “Computer Networking” by “James Kurose” TCP acronym for:– Transmission Control Protocol

● TCP together with UDP form the very core of today's Interenet transport layer.

● TCP is a protocol to send messages between two machines.

So what is the difference between TCP and UDP?

● As discussed, there are two ways to send messages..– UDP– TCP

● The difference between these protocols is that:– UDP is NOT guaranteed to deliver a packet

● It only does a “Best Effort” approach

– TCP is guaranteed to deliver a packet

How does TCP work?

• When explaining TCP assume that there are two nodes (Node A and B) and Node A is sending a packet to Node B utilizing TCP.

• High Level example of TCP under ideal circumstances– Node A sets a timer (TimeoutTimer)– Node A sends a packet (segment) to Node B

• This segment contains a unique sequence number

– Node B sends an Ack to Node A• Tells Node A that it received the msg.

– Node A’s TimeoutTimer expires• Does nothing since received Ack from Node B

– Node B stores the bytes it receives in a buffer• Application gets bytes from buffer

So what are some cases where TCP is not Ideal?

• In TCP a packet can be lost during several locations.– When the packet is transmitted from A to B– At Node B– When the packet it transmitted from B to A

How can you detect that packets have been lost?

● There are two ways.– Timeout– Triple acknowledgement

Timeout

• Node A knows it’s message is lost if:– It’s timer expires and it has

not received an Ack from Node B

• If a message is lost– Node A resets it’s timer– Node A resends the packet

• Same sequence number

• Process repeats

• Typically congestion is the reason messages are lost due to timeouts

Quick Discussion… Based on the discussion…. how is TCP a self tuning

algorithm.● In TCP if node A’s timer expires before an ack is

received it sets it’s timer for double it’s previous timer.– For instance, if Node A waits 10 seconds for an ack

for packet 1– After 10 seconds if node A receives no Ack– Node A retransmits packet A– Sets it timer for 20 seconds

● Once a packet an Ack received it sets it’s timer back to the original 10 secs.

Triple Acknowledgement

● Another way of determining packet loss● First things to note

– In TCP packets must be delivered in order– Also Node A (i.e. sender) can send multiple packets at

a time● Technically what is a triple acknowledgement

– When a sender receives multiple Acks for the same packet

Triple Acknowledgement Example

• Node A sends 5 packets• Node A never receives an

Ack for the second one• Node B receives packets 1,

3,4, 5• When node B receives

packets 3, 4, 5 send an ack that signifies that it never seen packet 2

• Node 2 receives 3 acks for the same packet it retransmits lost packet

How can you limit packet loss?

● What happens if Node A sends packets quicker then Node B processes the packet.– In this case Node B’s buffer overflows

● To ensure this does not happen TCP implements:– Flow control process

● Speed matching service… matching the rate at which the sender is sending to the rate at which the receiving application is reading

How is TCP implement flow control?

● Node B has a buffer– The available space varies depending on how many packets received it

received from Node A as well as the rate the Node B is processing the packet.

● Every time Node B sends a packet to Node A, Node B specifies the remaining size of it’s buffer

● Node A determines the number of packets it can send without overflowing node B’s buffer

● After Node A sends these packets it waits until it receives an Ack from Node B.

● Once it receives an Ack Node A can send another packet.● This window’s size varies over time.

Example of Window Size

Does dropped packets affect Congestion Control?

● Oops… not only can node B experience congestion but routers between Node A and B can experience congestion

– How do you adjust the rate A sends packets based on these also.

● If Node A experiences packet loss as a result of timeout– it cuts it’s window size in half.– It then incrementally increases it’s window size as it receives Acks. (slow start)– When the window size hits a threshold it grows it’s window exponentially

● If Node A experiences packet loss as a result of Triple Ack– It cuts it’s window size in half– It then incrementally increases it’s window size as it receives Acks. . (slow start)– When the window size hits window size hits ½ the value it had before the timeout it

exponentially grows the window

Quick Discussion… Again how is TCP a self tuning algorithm?

● As discussed TCP self tunes to alleviate congestion– I.e. in TCP a node’s window size fluctuates based on

the ability of Node B to process packets.

Reno Flavor TCP

● In this paper they only consider the Reno flavor of TCP– They argue it’s most widely used on the internet

● Difference between TCP and TCP reno.– Whenever Node A is in a slow start and receives a

triple Ack, it cancels the slow start and starts growing it’s window exponentially

● Known as fast recovery

Paper

● Paper Title: “Modeling TCP Throughput: A simple model and it’s empirical validation”

● Goals of paper:– Model TCP empirically (i.e. through a lot of math)– Predict throughput

● Previous work– People have previously simulated TCP in simulation

● Not sufficient as does not offer “TCP-friendly”, throughput for a non-TCP flow that interacts with a TCP connection…

– For instance FTPF interacting with TCP

Paper

● In this paper the authors state:– Their approach captures

● TCP’s fast retransmit mechanism (done in previous work)● TCP’s timeout mechanism on throughput (not done in

previous work)

– Their approach is able to accurately predict throughput over a significantly wider range of loss rate than before.

Assumptions

● They model TCP congestion avoidance behavior in terms of rounds – i.e. round starts with the back-to-back transmission of W

packets, where W is the current size of the TCP congestion widow.

● Once all packets in this window sent no further packets are sent until first Ack is received

● The duration of a round is equal to the round trip time and is assumed to be independent of the window size.– Also assume that the time it takes to send all packets in a

window is less then the round trip time.● A receiver sends one Ack for every two packets received

Assumptions (Cont)

● A packet is lost in a round independently of any packets lost in other rounds

● Packet loss is correlated among back to back transmissions in a round.– i.e. one packet in a round dropped all subsequent

packets in the round dropped.

Before we forget let’s analyze these assumptions based on what we know

about TCP.● Are these assumptions acceptable?

– They model TCP congestion avoidance behavior in terms of rounds ● Makes sense… TCP does the same thing

● The duration of a round is equal to the round trip time and is assumed to be independent of the window size.

– I would assume the window size does impact the round time since the window size dictates how many packets are sent..

● Hence this effects the length of the timeout timer since the last packet in the window’s timer will go off before the first packet in the widow’s timer.

● The time it takes to send all packets in a window is less then the round trip time.

– No necessarily.. A node could sets its timer to short and have to periodically adjust it as it further learns the time it takes to send a packet to the receiver

Assumption Analysis

● A receiver sends one Ack for every two packets received– Sounds good to me… I don’t know what the problem with this would be.

● A packet is lost in a round independently of any packets lost in other rounds

– Not valid.. ● What happens if node A sends packet 1 to node B in round 1 via Router C● This packet fills Router C’s queue● Node A realizes this packet is lost and resends this packet to B which B

receives and sends Ack which A receives● In another round Node A sends packet 2 to node B via Router C● Since packet 1 is still filling up router C’s buffer, this packet is dropped.● As you can see packet1 effects packet2

Assumption Analysis

● Packet loss is correlated among back to back transmissions in a round.– Very invalid…

● Node A sends in round 1 packet 1 to node B via router 1– Packet lost at router 1

● Node A then sends in round 1 packet 2 to node B via router 2

– Packet received by Node B ● Also don’t understand how you can get triple

acks if all subsequent packets are lost?– Anyone?

What do they develop empirical models for?

● Develop stochastic model of TCP congestion control in several steps, corresponding to it’s operating regimes– When loss indication are exclusively from triple

duplicate acks– When loss indications are both Timeout and triple

duplicate– When congestion window is limited by receiver’s

advertised window.● I don’t get into the math at all…

Measurements and Trace Analysis

● Experimental setup– Obtained data from 37 TCP connections established

between 18 hosts scattered across US and Europe● Operating systems at these hosts varied form win95-sun-

linux– Mostly flavors of linux

● TCP varies between approaches– In linux 2 acks instead of 3 for triple duplicate Ack

– All data sets are for unidirectional bulk transfers● i.e. FTP

Measurements and Trace analysis

• Example of data utilized in experiments– Shows overview of traces of

machine.

• They would like to point out how Timeout (TO) dominated the packets lost.

• This data is used to validate their models

Measurements and Trace analysis

• Broke trace data from 1 hour into 100 second intervals.

– Each point is a 100 sec interval• Plotted in lines

– Another papers model• Considers only triple duplicate acks

– Their model (proposed full)

• TD – no timeout only triple duplicate acks

• TO -> at least one timeout but no exponential backoff

• T1 -> experiences one exponential back off

– i.e. timeout after a timeout• T2 -> experience two exponential back

off• T3-> experience three exponential back

off

Measurements and Trace analysis

• Their approach better estimates observed model.– Their approach

• proposed (Approximate)

– Other approach • TD Only

Problems with their model

• Their approach is not always good– In this graph the other

model is a better estimator• They believe because in this

data a lot of duplicate acks are transpire

– Recall in the other paper they are only trying to estimate duplicate acks

– In this paper they are also estimating timeouts.. Etc…

Questions?