Congestion Control, the Internet Transport Protocols, and UDP

By: Mike Krupinski and Jaesma Woods

Scenario:

Angry driver

Other angry driver

What is congestion control:

• Controlling congestion relating to packet transfers over a network.

• If too many packets are sent at once, then you have a problem.

• Oh no.

How is this achieved?

1. Bandwidth should be properly allocated 2. Well measured goodput

Efficiency and Power:

• An efficient allocation of bandwidth uses all network capacity.

• Use of goodput (rate of useful packets arriving at receiver)

• As load increases, goodput increases

• Power will rise with the offered load.

•Power = load/delay•Proposed

by Kleinrock

Max/Min Fairness

• Definition: bandwidth given to one flow cannot be increased without decreasing the bandwidth given to another flow with an allocation that is no larger (Really long defintion).

Convergence and sending rate:

• The ideal operating point of a network varies over time.

• Sending rate may be limited by two factors: flow control and congestion

• Ex: A guy at the post office gets too much mail. He freaks out and storms out. A lot of mail is lost because of this. Plus, he hates his job.

Wireless Issues:

• High throughput requires very little packet loss• Sender may not be aware of wireless links if all

he knows is a wired link.• Internet paths are hectic because there is no

simple way to tell what links are being used.

Internet Transport Protocols: UDP

• Two main protocols:– Connectionless– Connection-oriented

• They complement one-another• The connectionless protocol is UDP, so it just

sends stuff.• The connection-oriented protocol is TCP, so it

does all the work. In other words, it’s the guy that takes up the slack.

What is UDP?

• An acronym for User Datagram Protocol– Provides a way for apps to send encapsulated IP

datagrams w/o a connection. • Transports segments consisting of 8-byte

header followed by payload• Ports act as identifiers for end points within

starting and ending machines

Source Port vs. Destination Port

• Source port is the incoming segment

• Destination port handles outgoing segment

• The UDP length of the packet includes header and data– Min is 8 bytes

• UDP DOES NOT:– Handle flow control– Handle congestion

control– Handle retransmission

Remote Procedure Call:

• Information can be transported from the caller to callee in the parameter and come back in procedure result– Calling procedure is known as client– Called procedure is known as server

• Calling remote procedures must have lib. Procedure known as client stub

• Server must have a procedure known as server stub.

Real Time Transport Protocols

• Examples would be Internet Radio, music-on-demand, videoconferencing, video-on demand.

RTP Contd.

• Purpose is to multiplex several real-time data streams on single streams of UDP packets.

• Packets aren’t treated by routers• Each packet is given higher number than last.• Incorporates time stamping– Associates time stamp with first sample in each

packet.

Questions Part One:

• 1. What is the formula for power?• 2. What are some wireless issues?• 3. What is UDP?• 4. What is RTP and what are some examples?• 5. What are the two main protocols of the

Internet?

Oh look!!! A video!!! Let’s watch!

http://www.youtube.com/watch?v=Ox8D2YBRIrQ

RTCP: Real time Transport Control Protocols

• Handles feedback, synchronization, and the user interface

• Does not transport media samples• First function is handling feedback on delay or

bandwidth congestion

Buffering and Jitter Control:

• Jitter is the variation in delay– Solution is to buffer the data

• Two solutions in scenarios:– Packet can be skipped– Playback can be paused until the packet arrives

• Ex: Watching a slow video on Youtube

Playback Point:

• How long to wait at the receiver for media before playing it out.– High jitter may need to be further out.

• Incorporation of talkspurts can assist with avoiding delay – Used in pauses in video/audio

Transmission Control Protocol:

• Designed to provided reliable end-to-end byte stream over an unreliable network.

• Different networks provide different layouts so this presents great challenges

• TCP must adapt to these changes

TCP Service Model

• End points in the model are known as sockets– Each socket has a number and a sixteen bit

number known as a port• A socket may be used for multiple connections

at the same time.• Port numbers below 1024 are reserved for

standard services.

Internet Daemons

• May attach itself to multiple ports and wait for first incoming connection

• System administrators often set up permanent daemons on ports

• All TCP connections are full duplex and point-to-point which means traffic can go in both directions or has exactly two end points.

Byte Streams:

• TCP connections are byte streams• Message boundaries are not preserved end to

end• It may split packets into chunks in various

numeric forms.• Files in UNIX often follow this concept

Urgent data:

• Focuses priority on a certain connection• Data is flagged with an Urgent Flag, which

prioritizes the data• Everything is transmitted into the particular

data.

TCP Segments and MTUs

• A TCP segment consists of a fixed twenty byte header– Followed by zero or more data bytes

• A MTU (Maximum Transfer Unit) is measured in 1500 bytes– Defines upper bound on segment size

TCP Connection Establishment:

• Server• Client• Destination• These tree must have a

three-way agreement between one another.

TCP Sliding Window:

• Measured in how much data is buffered for the sender so he can send more data.

• Window probe: Used to prevent deadlock if a window update is ever lost.

• Senders are not required to send data as soon as they come from application.

TCP Congestion Control:

• Congestion occurs when load is too much to handle

• TCP maintains congestion window, whose size is the number of bytes the sender may have in the network.

• Congestion collapse occurs when goodput drops precipitously.

TCP Implementation:

• TCP uses selective acknowledgements known as SACK, which lists ranges of received bytes.– Allows sender to more directly decide what

packets to retransmit. • SACK is strictly advisory information. – TCP can use SACK to recover more easily from

situations.

Questions Part 2:

• What is SACK?• What is Urgent Data?• What does RTCP Stand For?• What is Buffering?

Hooray. You survived.