Governing Communication

Winter 2005

CMPE 151: Network Administration

Lecture 2

Winter 2005

Review? Network protocols. TCP/IP.

Winter 2005

Outline Network protocols. IP TCP

Winter 2005

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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Internetworking

Winter 2005

Internetworking Interconnection of 2 or more

networks forming an internetwork, or internet. LANs, MANs, and WANs.

Different networks mean different protocols. TCP/IP, IBM’s SNA, DEC’s DECnet,

ATM, Novell and AppleTalk.

Winter 2005

Internetworks (cont’d)

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TCP/IP

• TCP/IP is the most widely used internetworking protocol suite– Initially funded through ARPA.– Picked up by NSF.– Used in the Internet.

• Other internetworking protocols exist but are less used– Example: AppleTalk, X.25, etc.

Winter 2005

IP

Winter 2005

The Internet Protocol: IP Glues Internet together. Common network-layer protocol

spoken by all Internet participating networks.

Best effort datagram service: No reliability guarantees. No ordering guarantees.

Winter 2005

IP (cont’d)

• IP is responsible for datagram routing.• Important: each datagram is routed

independently!– Two different datagrams from same source to same

destination can take different routes!– Why?– Implications?

Winter 2005

IP (cont’d)

• IP provides a best effort delivery mechanism– Does not guarantee to prevent duplicate

datagrams, delayed and out-of-order delivery, corruption of data or datagram loss

• Reliable delivery is provided by the transport layer, not the network layer (IP)

• Network layer (IP) can detect and report errors without actually fixing them

Winter 2005

The Internet Protocol

Router Router

Host Host

Application

Transport

Network

IP IPIP IP

Network

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Datagrams Transport layer breaks data

streams into datagrams which are transmitted over Internet, possibly being fragmented.

When all datagram fragments arrive at destination, reassembled by network layer and delivered to transport layer at destination host.

Winter 2005

IP Datagram Format IP datagram consists of header and

data (or payload). Header:

20-byte fixed (mandatory) part. Variable length optional part.

Winter 2005

IP Versions IPv4: IP version 4.

Current, predominant version. 32-bit long addresses.

IPv6: IP version 6. Evolution of IPv4. Longer addresses (16-byte long).

Winter 2005

IP(v4) Header FormatHeader

Payload

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Encapsulation

• Each datagram is encapsulated within a data link layer frame– The whole datagram is placed in the data area of

the frame.– The data link layer addresses for source and

destination included in the frame header.

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Encapsulation - Example

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Encapsulation Across Multiple Hops

• Each router in the path from source to destination:– Decapsulates datagram from incoming frame.– Forwards datagram - determines next hop.– Encapsulate datagram in outgoing frame.

Winter 2005

Encapsulation Across Multiple Hops - Example

Winter 2005

Maximum Transfer Unit

• Each data link layer technology specifies the maximum size of a frame.– Called the Maximum Transfer Unit (MTU).

• Ethernet: 1,500 bytes.• Token Ring: 2048 or 4096 bytes.

• What happens when large packet wants to travel through network with smaller MTU?• Maximum payloads (data portion of datagram)

range from 48 bytes (ATM cells) to 64Kbytes (IP packets).

Winter 2005

Fragmentation

• Another solution (used by IP): fragmentation.• Gateways break packets into fragments to fit the

network’s MTU; each sent as separate datagram.• Gateway on the other side have to reassemble

fragments into original datagram.

Winter 2005

Keeping Track of Fragments

Fragments must be numbered so that original data stream can be reconstructed.

Define elementary fragment size that can pass through every network.

When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller).

Datagram may contain several fragments.

Winter 2005

Fragmentation - Example

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Addressing

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Universal Addressing

• One key aspect of internetworks is unique addresses.

• Sending host puts destination internetworking address in the packet.

• Destination addresses can be interpreted by any intermediate router/gateway.

• Router/gateway examines address and forwards packet on to the destination.

Winter 2005

IP Addresses• Each machine on the Internet has a unique IP address.• The IP address is different from the “physical” /“MAC”

address.– The “physical address” is the address of a computer

(actually, of a NIC) in the LAN.• It is only know within the LAN.

– The IP address is a universal address.

– When a packet arrives in a LAN, there needs to be a conversion from IP to MAC address (local “address resolution”).

Winter 2005

IP Addresses (cont’d)

• An IP address is represented by a binary number with 32 bits (in IPv4).– Meaning that there are around 4 billion

addresses.– Often IP addresses are represented in “dotted

decimal”, such as 128.114.144.4.• Each group of numbers can go from 0 to 255.

Winter 2005

IP Address Organization

• Each IP address is divided into a prefix and a suffix– Prefix identifies network to which computers

are attached.– Suffix identifies computers within that

network.

Winter 2005

Network and Host Numbers

• Every network in a TCP/IP internet is assigned a unique network number.

• Each host on a specific network is assigned a host address that is unique within that network.

• Host’s IP address is the combination of the network number (prefix) and host address (suffix).

• Assignment of network numbers must be coordinated globally; assignment of host addresses can be managed locally.

Winter 2005

IP Address Format

• IP address are 32 bits long.• There are different classes of addresses,

corresponding to different subdivisions of the 32 bits into prefix and suffix.– Some address classes have large prefix, small

suffix.• Many such networks, few hosts per network.

– Other address classes have small prefix, large suffix.

• Few such networks, many hosts per network.

Winter 2005

IP Address Format (cont’d)• How can we recognize to which class an IP

address belongs to?– Look at the first 4 bits!

Winter 2005

IP Address Format (cont’d)

• Class A, B and C are primary classes.– Used for ordinary addressing.

• Class D is used for multicast, which is a limited form of broadcast.– Internet hosts join a multicast group.– Packets are delivered to all members of the

group.– Routers manage delivery of single packets

from source to all members of multicast group.

• Class E is reserved.

Winter 2005

IP Addresses (cont’d)

• Another way to determine the address class is by looking at the first group of numbers in the dotted decimal notation

Winter 2005

Networks and Hosts in Each Class

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Understanding IP Addresses

• Examples:– 10.0.0.37 (class A)

– 128.10.0.1 (class B)

– 192.5.48.3 (class C)

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IP addresses: how to get one?

• ICANN (Internet Corporation for Assigned Names and Numbers) coordinate IP address assignment.

• How does host get its IP address in the network? 2 possibilities:– 1: Hard-coded by system administrator in a file

inside the host.– 2: DHCP: “Dynamic Host Configuration Protocol”

• Dynamically get address: “plug-and-play”.

Winter 2005

DHCP

• DHCP allows a computer to join a new network and automatically obtain an IP address The network administrator establishes a pool of addresses for DHCP to assign.

• When a computer boots, it broadcasts a DHCP request to which a server sends a DHCP reply.

Winter 2005

DHCP (Cont’d)• DHCP allows non-mobile computers that run

server software to be assigned a permanent address (won’t change when the computer reboots).

– The permanent address actually needs to be re-negotiated after a certain period of time.

Winter 2005

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.

Winter 2005

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.

Winter 2005

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.

Winter 2005

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

Winter 2005

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.

Winter 2005

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.

Winter 2005

TCP Protocol Overview 2 Sequence numbers.

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

Winter 2005

TCP Connection Setup 3-way handshake.

Host 1 Host 2SYN (SEQ=x)

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

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

Winter 2005

TCP Connection Release 1 Abrupt release:

Send RESET. May cause data loss.

Winter 2005

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.

Winter 2005

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.

Winter 2005

TCP Transmission 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.

Winter 2005

TCP Transmission (cont’d) 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.

Winter 2005

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.

Winter 2005

TCP Flow Control: Example2K;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

Winter 2005

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.

Winter 2005

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.

Winter 2005

TCP Congestion Control 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).

Winter 2005

TCP Congestion Control (cont’d) 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.

Winter 2005

TCP Congestion Control (cont’d) 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.

Winter 2005

TCP Congestion Control: Example

threshold

timeout

threshold

cwin

time

Winter 2005

TCP Retransmission Timer When segment sent,

retransmission timer starts. If segment ACKed, timer stops. If time out, segment retransmitted

and timer starts again.

Winter 2005

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.

Winter 2005

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.

Winter 2005

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.

Winter 2005

Client-Server Model

Client

Kernel

File Server

Kernel

Printer Server

Kernel

Winter 2005

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.

Winter 2005

The Web and HTTP

Winter 2005

The Web WWW, or the world-wide web is a

resource discovery service. Resource space is organized

hierarchically, and resources are linked to one another according to some relation.

Hypertext organization: link “granularity”; allows links within documents.

Graphical user interface.

Winter 2005

The Client Side Users perceive the Web as a vast

collection of information. Page is the Web’s information transfer unit. Each page may contain links to other

pages. Users follow links by clicking on them which

takes them to the corresponding page. This process can go on indefinetly,

traversing several pages located in different places.

Winter 2005

The Browser Program running on client that retrieves

and displays pages. Interacts with server of page. Interprets commands and displays page.

Examples: Mosaic, Netscape’s Navigator and Communicator, Microsoft Internet Explorer.

Other features: back, forward, bookmark, caching, handle multimedia objects.

Winter 2005

Domain Name System (DNS) Basic function: translation of

names (ASCII strings) to network (IP) addresses and vice-versa.

Example: zephyr.isi.edu <-> 128.9.160.160

Winter 2005

DNS Hierarchical name space. Distributed database. RFCs 1034 and 1035.

Winter 2005

How is it used? Client-server model.

Client DNS (running on client hosts), or resolver.

Application calls resolver with name. Resolver contacts local DNS server

(using UDP) passing the name. Server returns corresponding IP

address.

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Name Resolution 1 Application wants to resolve name. Resolver sends query to local name server.

Resolver configured with list of local name servers.

Select servers in round-robin fashion. If name is local, local name server returns

matching authoritative RRs. Authoritative RR comes from authority

managing the RR and is always correct. Cached RRs may be out of date.

Winter 2005

Name Resolution 2 If information not available locally

(not even cached), local NS will have to ask someone else. It asks the server of the top-level

domain of the name requested.

Winter 2005

Electronic Mail Non-interactive.

Deferred mail (e.g., destination temporarily unavailable).

Spooling: Message delivery as background

activity. Mail spool: temporary storage area

for outgoing mail.

Winter 2005

Mail system

Userinterface

Usersends mail

Userreads mail

Outgoingmailspool

Mailboxes incomingmail

Client(send)

Server(receive)

TCPconnection(outgoing)

TCPconnection(incoming)

Winter 2005

Observations When user sends mail, message

stored is system spool area. Client transfer runs on background. Initiates transfer to remote

machine. If transfer succeeds, local copy of

message removed; otherwise, tries again later (30 min) for a maximum interval (3 days).

Winter 2005

Remote access

Winter 2005

Telnet

User’smachine

Telnetclient

OSTCP connectionover Internet

Telnetserver

OS

Winter 2005

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.