CMPT 471 Networking II

1

CMPT 471Networking II

Some important Internetworkingconcepts

© Janice Regan, 2013

© Janice Regan, 2013 2


Internetworking Requirements Provides linkage between networks

Minimum physical and link layer Provides routing and delivery of data between

processes on different networks Provides accounting services and maintains

status information Supports services for users and the network Provides these services independent of

individual network architectures, and must accommodate differences between networks


Architectural Approaches: 1 Connection oriented (reliable) delivery service

Delivered using a sustained connection across the network between two machines

The connection is initiated or built before it can be used The connection, once established delivers data streams

between the endpoints of the connection (a full duplex connection will have 1 data stream in each direction)

The connection is terminated when the application is finished using it

Resources for the circuit are allocated to connection. If circuit is idle then resources are unused and unavailable to other circuits


Architectural Approaches: 2 Connectionless

Stream of data is divided into pieces Each piece is encapsulated in a packet by adding a

header containing all information needed to get the packet to its destination

Each packet is sent independently and is individually routed through the network

Different packets from the same data stream may take different paths through the internet

Packets may arrive in an order different from the order in which they were sent (different paths = different travel times)


Architectural Approaches: 1 Connection oriented (reliable) delivery services

Support for quality of service simple Failure of any router in the circuit will cause failure of

the connection Path setup overhead but reduced routing overhead

(routing done once not for each packet) Router table space needed for every connection

passing through the router (may need less table space for address for a connection, but will need an entry for each connection not each network)

All networks involved must be circuit switched networks (pass through switching)


Architectural Approaches: 2 Connectionless

Support for quality of service is difficult Delivery will not fail if a single

intermediate router fails Extra overhead in routing, must

separately route each packet No congestion control available Best effort (not reliable) transmission of

packets


Connectionless Internetworking Advantages

Flexible and Robust Minimal overhead (no connection overhead)

Disadvantages Delivery not guaranteed Order of delivery not guaranteed Reliability is responsibility of next layer up

(e.g. TCP, transport layer builds a virtual circuit)


Internetworking Protocols



Encapsulation Data is transferred in blocks called Protocol

Data Units (PDU) Each PDU contains control information and

sometimes data Control Information includes

Address Error Detection code Protocol Control

The addition of control information to data is referred to as Encapsulation

© Janice Regan, 2013 12Figure 2.14 Stallings (2003)

Transport layer

Network layer

Data link layer (e.g. ethernet)

port

ip address

MAC address

Encapsulation: Protocol Data Units in the TCP/IP Architecture


Protocol functions: IP (network) layer

Routing Determining a path through the Internet through a series of

hosts/routers to the destination Addressing/ Forwarding

Sending packets along the determined path Datagram lifetime

Removing packets that are ‘lost’ or ‘damaged’ Fragmentation and re-assembly

Dealing with networks having different maximum datagram sizes

Error control, Flow control Connection management (if needed) Transmission services


Addressing: Each Internet Protocol address (IP address)

must be globally applicable and globally unique Must have global nonambiguity, must uniquely

identify the host and interface globally Must have global applicablity, must be accessible

via address from any other node on the Internet IP addresses are used to route PDU from the

destination through ISs to the destination. Routes the PDU through the internet or Internet.


Internet addresses IPv4 32 bit global internet (IPv4) address is used to uniquely

identify a particular network interface connected to a particular host as a destination for communication

Each IPv4 address is split into two parts (netid, hostid) to identify the host and the network to which the host is connected

The netid (network address or prefix) identifies the network to which the host belongs.

The hostid identifies the particular host (network interface for a multi homed host)


Internet addresses IPv6 128 bit global (IPv6) address is used to uniquely identify

a particular network interface connected to a particular host as a destination for communication

Each IPv6 address is split into three parts (global routing prefix, subnet ID, interface identifier)

The global routing prefix (48 bits) identifies the network to which the host belongs (like an IPv4 netid).

The subnet ID (16 bits) is used to create a subnet structure within the network

The interface identifier (64 bits) indicates the particular host (like IPv4 host ID)


Addressing: connection identifiers For a connection oriented network, a

connection identifier may be used rather than a global address

The connection identifier is smaller (fewer bits) and creates less overhead

Connection identifier indicates route to Intermediate stations

End systems maintain flow and state information for the connection identified


Deciding where to go Need to determine a path for a packet (or message) to

take through a network Two major components are required to determine such a

path Routing

Building a routing table for each host and router.

Forwarding: Using the routing tables at each IS to determine the most

efficient path to the destination.


Where to go? Routing Routing: Building a routing table for each host

and router. A routing table indicates which of the router’s ( host’s)

network interfaces provides the most direct path to each possible IP destination address

The entries in a routing table are determined statically (input by administrator) or dynamically using time varying routing data sent between routers and a routing algorithm


Where to go? Forwarding Forwarding:

Using the routing tables at each intermediate router to determine the most efficient path to the destination.

At each intermediate router find the ‘best’ interface to send the packet through.

Simple algorithm for forwarding Each row of routing table represents one network (or host).

For each row determine if the destination address is part of the network

If it is add that row to the list of possible candidates for forwarding

Choose the ‘best’ (longest match) of the candidates for forwarding

Use the information in the row for the ‘best’ network to forward the packet


A sample routing table IPv4Destination Gateway Genmask Metric Iface

172.17.1.253 * 255.255.255.255 0 Eth1

172.16.1.253 * 255.255.255.255 0 Eth0

172.16.0.0 * 255.255.0.0 0 Eth0

172.17.0.0 * 255.255.0.0 0 Eth1

172.18.0.0 172.16.1.254 255.255.0.0 1 Eth0

172.19.0.0 172.17.1.2 255.255.0.0 1 Eth0

127.0.0.0 * 255.0.0.0 0 Lo

224.0.0.0 * 240.0.0.0 0 Eth0

0.0.0.0 172.16.1.1 0.0.0.0 0 Eth0

A sample routing table IPv6


Destination Next Hop Flag Met Ref Use If::1/128 :: Un 0 1 41 lofdd0:8184:d967:25::a19:106/128 fdd0:8184:d967:29::a1d:109 UG 1024 0 203 eth1fdd0:8184:d967:26::/64 fdd0:8184:d967:30::a1e:108 UG 1024 0 122 eth2fdd0:8184:d967:29::/128 :: Un 0 2 0 lofdd0:8184:d967:29::a1d:10a/128 :: Un 0 1 350 lofdd0:8184:d967:29::/64 :: U 256 0 1 eth1fdd0:8184:d967:30::/128 :: Un 0 2 0 lofdd0:8184:d967:30::a1e:10a/128 :: Un 0 1 392 lofdd0:8184:d967:30::/64 :: U 256 0 1 eth2 fe80::250:56ff:fea4:1795/128 :: Un 0 1 74 lofe80::250:56ff:fea4:59c9/128 :: Un 0 1 63 lofe80::250:56ff:fea4:764f/128 :: Un 0 1 0 lofe80::/64 :: U 256 0 0 eth1fe80::/64 :: U 256 0 0 eth2fe80::/64 :: U 256 0 0 eth0::/0


Datagram Lifetime Datagrams could loop indefinitely

Problems with routing or addressing can cause datagrams to travel in circles or on circuitous paths rather than reach their destination promptly

Consumes resources Internet protocol may need upper bound on datagram life

Datagram marked with lifetime Time To Live field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count: Decrement time to live on passing through a each

router Time count: Need to know how long since last router


Fragmentation / Reassembly Each protocol has a maximum size for it’s own

PDUs. This maximum is different for different protocols

The data in the PDU must be broken into smaller pieces or fragmented if the PDU is too large for a network it passes through

The data in the fragmented PDU is reassembled, usually at the receiver


Why fragment? A PDU from another network exceeds the maximum

allowed size for the next network through which it will travel

To improve efficiency or error control. Fewer bits need to be retransmitted (reduce PDU size)

More equitable sharing of facilities Reduced buffers needed at stations But remember the costs

Increased overhead of control information More interrupts, may increase processing time

26

CMPT 471Networking II

Review of TCP/UDP The transport layer

© Janice Regan, 2013


The transport layer Processes use a transport protocol communicate end to

end. (reliably using TCP or best effort using UDP) Data-link and network layer protocols communicate hop

by hop, forming a communications channel for the transport layer

The transport layer shields the application processes from the details of the underlying communications channel

The transport layer specifies a particular application process as the source or destination. Lower layers specify at most the protocol in the next layer up



Transport protocols May provide best effort delivery of packets (UDP)

Application accepts responsibility for reliability May provide a connection oriented reliable

communication channel (TCP) either On top of a best effort packet switched network (IP with

unacknowledged connectionless LLC layer) On top of a reliable network service (IP with acknowledged

connectionless services) May support multiplexing (particularly useful for circuit

switch or virtual circuit networks using flows) Combine multiple low rate flows through 1 port Spread high rate flow between multiple ports

UDP: best effort transmission Addressing Multiplexing

Using one UDP port for all UDP traffic Error control

What happens when an error occurs Usually the packet is lost

Ordering Reordering received packets to be in same order they were

sent



Multiplexing of flows


TCP: reliable transmission Addressing Multiplexing Flow control Logical connection establishment Logical connection maintenance Logical connection termination error control reorder packets received the application


Addressing User specified by:

User identification Specify Host and Port (may be socket in TCP or UDP) Port represents particular process or protocol Host is specified by an IP address Several processes on a given machine may be multiplexed

together and use a single protocol port Protocol port identification specified by an integer

usually one protocol port for of each protocol type (TCP, UDP)

General port identification specified by an integer


Finding Port Addresses: 1 Static configuration:

Know address ahead of time Universal assignment:

Central authority (IANA) assigns port numbers to common protocols. The list is published

All software uses this list of protocols/ports Any host receiving a particular protocol will have a

server process for that protocol which can be accessed through the assigned protocol port number

Ports 0-1023 are reserved for this IANA list Ports 1024-49151 are registered (use list exists) not

reserved


Finding Port Addresses: 2 Name server: Server on a well known port

can be queried to determine the port for the particular application

Instance server: server on a well know port will create an instance of the application and return the appropriate port to attach to that instance


Well Know port numbers


Structure of a TCP segment

Comer 2000: fig 13.7

SOURCE PORT DESTINATION PORT

CHECKSUM


Code Bits (1) The code bits field contains a set of 6 flags

The ACK SYN and FIN flags produce packets used in the three way handshake to establish a TCP connection.


Code Bits (2) ACK must be set if the frame contains a

valid acknowledgement PSH requests that data be sent immediately

rather than waiting for enough data to fill a packet

When the urgent flag is set the value of the urgent pointer field added to the sequence number indicates the end (last octet) of a block of data that needs immediate delivery. Flag stays set until urgent data is delivered. The start of the urgent data is not marked.


Structure of a TCP segment

Comer 2000: fig 13.7

SEQUENCE NUMBER

ACKNOWLEDGEMENT NUMBER

WINDOW


Sequence number (1) A TCP connection is a full duplex (capable of sending

data in both direction simultaneously) connection with data flowing in both directions

A TCP segment contains both a TCP header and a variable number of data octets

Consider two data streams, the data going from host1 to host2 and the data going from host2 to host1

Separately consider each of these data streams. In each data stream each octet of data has an octet number. The octet number of the next octet in the stream is one larger than the octet number of the present octet


Sequence number (2)Stream of octets of data from host 1 to host 2

Stream divided to show which data octets are placed in each successive segment

1520 1420 1381 1281 1181 1100 1000 First data octet number to be placed in segment

Stream of octets of data from host 2 to host 1

Stream divided to show which data octets are placed in each successive segment

6000 5800 5600 5400 5300 5150 5000

First data octet number to be placed in segment

Two Different Streams of DATA


Sequence number (3) The octet count in each stream is independent.

Separate counters! TCP sequence numbers do not count packets

they count octets of data. This is because TCP segments can contain variable numbers of octets of data.

A TCP segment whose first data octet is octet m, has a sequence number m when it is sent


Acknowledgement numbers Consider the data stream from host1 to host2 When host1 sends a segment the sequence number of that

segment refers to octet number of the first octet in that segment. This octet is part of the stream of data from host1 to host2 and the sequence number refers to that stream.

When host1 receives a segment with the ACK flag set the segment carries a valid acknowledgement The acknowledgement number of a valid

acknowledgement being received by host1 indicates the octet number, m, of the next data octet that host2 expects to receive from host1 (ACKs receipt of all data octets up to octet m-1)


Piggybacking A valid acknowledgment can be sent in its own

packet (no data being sent from host2 to host1 in the segment)

A valid acknowledgement can be piggybacked. the acknowledgement is added to a segment

carrying data from host2 to host1 The acknowledgement is added by

setting the ACK flag of that segment placing the correct value in the acknowledgement number

field. The correct value is the octet number of the next octet of data expected. If the last octet received in the packet being acknowledged is M the correct value is M+1


Flow control + Error control Flow Control

Control flow of data from source to receiver Source sends at a particular rate (frames/sec) Receiver must be able to receive and process

data at that rate (rate must be controlled) Error control

How to recover when a frame is damaged or lost in transmission


Stop and Wait: Error free Packets arrive at

destination in the same order they are sent

Error free transmission

Send F2

Send F3

Send F1

Send F0

Send ACK1

Send ACK2

Send ACK3RTT

TI

ME


Stop and Wait Flow Control Source entity (sender) transmits frame and

starts a timer Destination entity (receiver) receives frame Destination entity sends ACK Source entity receives ACK and stops timer Source entity is now ready to start the cycle

again for a new frame If timer expires first four steps above are

repeated for the same frame

Error control How does the protocol recover when a

packet or an ACK is lost or damaged?

How can a packet or ACK be lost or damaged?

Will discuss specific cases later


Documents

CMPT 471 Networking II