Winter 2002 1

CMPE 155

Week 3

Winter 2002 2

Project 3: Basic Servers

Telnet Rlogin FTP Web

In this context, let’s look at the underlying protocols…

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Client-Server Model

Client

Kernel

File Server

Kernel

Printer Server

Kernel

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What are protocols?

Set of rules governing communication between network elements (applications, hosts, routers).

Protocols define:– Format and order of messages.– Actions taken on receipt of a message.

Protocols are hard to design– We need design guidelines!

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Protocol stack

Host Host

Application

Transport

Network

Link

User A User BTeleconferencing

Layering: technique to simplify complex systems

Peers

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Layering Characteristics

Each layer relies on services from layer below and exports services to layer above.

Interface defines interaction, Hides implementation - layers can

change without disturbing other layers (black box).

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Encapsulation

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OSI Model: 7 Protocol Layers

Physical: how to transmit bits Data link: how to transmit frames Network: how to route packets hop2hop Transport: how to send packets end2end Session: how to tie flows together Presentation: byte ordering, security Application: everything else!

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Layering Functionality

Reliability Flow control Fragmentation Multiplexing Connection setup (handshaking) Addressing/naming (locating peers)

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Example: Transport layer

First end-to-end layer. End-to-end state. May provide reliability, flow and

congestion control.

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Example: Network Layer

Point-to-point communication. Network and host addressing. Routing.

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The Internet Protocol

Router Router

Host Host

Application

Transport

Network

IP IPIP IP

Network

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IP

Glues Internet together. Common network-layer protocol spoken

by all Internet participating networks. Best effort datagram service:

– No reliability guarantees.– No ordering guarantees.

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Remote login protocols

Telnet and rlogin. Allow interactive use of remote

machines. Use reliable transport protocols, e.g.,

TCP.

What’s TCP?

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The Internet Transport Protocols: TCP and UDP UDP: user datagram protocol (RFC

768).– Connection-less protocol.

TCP: transmission control protocol (RFCs 793, 1122, 1323).– Connection-oriented protocol.

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UDP Provides connection-less, unreliable service.

– No delivery guarantees.– No ordering guarantees.– No duplicate detection.

Low overhead.– No connection establishment/teardown.

Suitable for short-lived connections.– Example: client-server applications.

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TCP

Reliable end-to-end communication. TCP transport entity:

– Runs on machine that supports TCP.– Interfaces to the IP layer.– Manages TCP streams.

• Accepts user data, breaks it down and sends it as separate IP datagrams.

• At receiver, reconstructs original byte stream from IP datagrams.

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TCP Reliability

Reliable delivery.– ACKs.– Timeouts and retransmissions.

Ordered delivery.

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TCP Service Model 1

Obtained by creating TCP end points.– Example: UNIX sockets.– Socket number or address: IP address + 16-bit

port number (TSAP).– Multiple connections can terminate at same

socket.– Connections identified by socket ids at both ends.– Port numbers below 1024: well-known ports

reserved for standard services.• List of well-known ports in RFC 1700.

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TCP Service Model 2

TCP connections are full-duplex and point-to-point.

Byte stream (not message stream).– Message boundaries are not preserved

e2e.

A B C D

4 512-byte segments sent asseparate IP datagrams

A B C D

2048 bytes of data deliveredto application in single READ

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TCP Byte Stream When application passes data to TCP, it may

send it immediately or buffer it. Sometimes application wants to send data

immediately.– Example: interactive applications.– Use PUSH flag to force transmission.– TCP could still bundle PUSH data together (e.g., if it

cannot transmit it right away).

URGENT flag.– Also forces TCP to transmit at once.

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TCP Protocol Overview 1

TCP’s TPDU: segment.– 20-byte header + options.– Data.

TCP entity decides the size of segment.– 2 limits: 64KByte IP payload and MTU.– Segments that are too large are fragmented.

• More overhead by addition of IP header.

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TCP Protocol Overview 2

Sequence numbers.– Reliability, ordering, and flow control.– Assigned to every byte.– 32-bit sequence numbers.

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TCP Connection Setup

3-way handshake.Host 1 Host 2SYN (SEQ=x)

SYN(SEQ=y,ACK=x+1)

(SEQ=x+1, ACK=y+1)

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TCP Connection Release 1 Abrupt release:

– Send RESET.– May cause data loss.

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TCP Connection Release 2 Graceful release:

– Each side of the connection released independently.

• Either side send TCP segment with FIN=1.• When FIN acknowledged, that direction is shut down for

data.• Connection released when both sides shut down.

– 4 segments: 1 FIN and 1 ACK for each direction; 1st. ACK+2nd. FIN combined.

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TCP Connection Release 3

Timers to avoid 2-army problem.– If response to FIN not received within

2*MSL (maximum segment lifetime), FIN sender releases connection.

After connection released, TCP waits for 2*MSL (e.g., 120 sec) to ensure all old segments have aged.

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TCP Transmission 1

Sender process initiates connection. Once connection established, TCP can

start sending data. Sender writes bytes to TCP stream. TCP sender breaks byte stream into

segments.– Each byte assigned sequence number.– Segment sent and timer started.

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TCP Transmission 2 If timer expires, retransmit segment.

– After retransmitting segment for maximum number of times, assumes connection is dead and closes it.

If user aborts connection, sending TCP flushes its buffers and sends RESET segment.

Receiving TCP decides when to pass received data to upper layer.

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TCP Flow Control

Sliding window.– Receiver’s advertised window.

• Size of advertised window related to receiver’s buffer space.

• Sender can send data up to receiver’s advertised window.

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TCP Flow Control: Example

2K;SEQ=0

ACK=2048; WIN=2048

2K; SEQ=2048

ACK=4096; WIN=0

ACK=4096; WIN=2048

1K; SEQ=4096

App. writes 2K of data

4K

2K

0

App. reads 2K of data

2K

1K

App. does 3K write

Senderblocked

Sendermay send upto 2K

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TCP Flow Control: Observations

TCP sender not required to transmit data as soon as it comes in from application.– Example: when first 2KB of data comes in,

could wait for more data since window is 4KB.

Receiver not required to send ACKs as soon as possible.– Wait for data so ACK is piggybacked.

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Congestion Control

Why do it at the transport layer?– Real fix to congestion is to slow down sender.

Use law of “conservation of packets”.– Keep number of packets in the network constant.– Don’t inject new packet until old one leaves.

Congestion indicator: packet loss.

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TCP Congestion Control 1

Like, flow control, also window based.– Sender keeps congestion window (cwin).– Each sender keeps 2 windows: receiver’s

advertised window and congestion window.– Number of bytes that may be sent is

min(advertised window, cwin).

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TCP Congestion Control 2

Slow start [Jacobson 1988]:– Connection’s congestion window starts at 1

segment.– If segment ACKed before time out,

cwin=cwin+1.– As ACKs come in, current cwin is

increased by 1.– Exponential increase.

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TCP Congestion Control 3

Congestion Avoidance:– Third parameter: threshold.– Initially set to 64KB.– If timeout, threshold=cwin/2 and cwin=1.– Re-enters slow-start until cwin=threshold.– Then, cwin grows linearly until it reaches

receiver’s advertised window.

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TCP Congestion Control: Example

threshold

timeout

threshold

cwin

time

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TCP Retransmission Timer

When segment sent, retransmission timer starts.– If segment ACKed, timer stops.– If time out, segment retransmitted and

timer starts again.

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How to set timer?

Based on round-trip time: time between a segment is sent and ACK comes back.

If timer is too short, unnecessary retransmissions.

If timer is too long, long retransmission delay.

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Jacobson’s Algorithm 1

Determining the round-trip time:– TCP keeps RTT variable. – When segment sent, TCP measures how

long it takes to get ACK back (M).– RTT = alpha*RTT + (1-alpha)M.– alpha: smoothing factor; determines weight

given to previous estimate.– Typically, alpha=7/8.

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Jacobson’s Algorithm 2

Determining timeout value:– Measure RTT variation, or |RTT-M|.– Keeps smoothed value of cumulative

variation D=alpha*D+(1-alpha)|RTT-M|.– Alpha may or may not be the same as

value used to smooth RTT.– Timeout = RTT+4*D.

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Keepalive Timer

Goes off when a connection is idle for a long time.

Causes one side to check whether the other side is still alive.

If no answer, connection terminated.

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TIME_WAIT

2*MSL. Makes sure all segments die after

connection is closed.

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Back to remote login…

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Telnet

User’smachine

Telnetclient

OSTCP connectionover Internet

Telnetserver

OS

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Telnet basic operation

When user invokes telnet, telnet client on user machine establishes TCP connection to specified server.

TCP connection established; user’s keystrokes sent to remote machine.

Telnet server sends back response, echoed on user’s terminal.

Telnet server can accept multiple concurrent connections.

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Handling heterogeneity

Telnet protocol specifies standard data exchange: network virtual terminal (NVT).

Telnet client and server make translation.

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Rlogin

Remote login between Unix hosts. Simpler than telnet.

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More details…

Stevens, TCP/IP Illustrated, Vol. 1 Comer, Internetworking with TCP/IP,

Vol. 1, 4th. edition. RFC 854 (Telnet).

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File Transfer

Sharing remote files: “on-line” access versus “file transfer”.

“On-line” access transparent access to shared files, e.g., distributed file system.

Sharing through file transfer: user copies file then operates on it.

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FTP

File transfer accounted for most of the Internet traffic until the Web exploded!

Also uses TCP. Allows interactive access; format

specification (e.g., binary); authentication (clients required to authenticate themselves).

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FTP Operation Client Server

OS

Data Control

OS

Data Control

TCP connection

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Port Assignment

FTP server listens on well-known port (21); data transfer uses port 20.

On client side, uses any unused port; client control process communicates that port number to server.

Server process initiates data transfer connection.

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Anonymous FTP