• Review the key networking concepts– TCP/IP reference model– Ethernet – Switched Ethernet– IP, ARP– TCP– DNS

FTP data can be infinitely large, when do we chop it into smallerUnits?

• Internetworking devices– Repeaters: PHY device that restores data and collision

signals

– Hub: Multiport repeater

– Bridge: Datalink layer device connecting two or more collision domains (extended LAN). MAC multicasts are propagated throughout the extended LAN

– Switch: Multiport bridge with parallel paths

– Router: Network layer device.

– Network address translator (NAT): Transport layer device.

– Gateways: Transport layer or application layer devices

• Ethernet– Shared medium LAN technology– Need medium access control

• CSMA/CD + binary exponential backoff

– Frame format:

|Preamble|Start|Dst Addr|Src Addr|length| Data | Pad |Checksum 7 1 6 6 2 0-1500 0-46 4

• Minimum data + pad length: 46 bytes

• Maximum data length: 1500 bytes

• Fast Ethernet (100Mbps)– Reduce cable length and keep everything else– Duplex mode

• Gigabit Ethernet (1000Mbps)– Reduce cable length and increase minimum

frame size– Duplex mode

• Switched Ethernet– Get dedicated link in Ethernet in a transparent

manner.– Issue: the switch needs to know where to

forward a packet– How is it accomplished?

• Self-learning algorithm to obtain the location of each machine

• Forwarding algorithm

– Can switched Ethernet take advantage of the dense connectivity in the network topology?

• Spanning tree algorithm

• IP– Addressing

(type, network number, host number)

• Class A, Class B, Class C

– Classless inter-domain routing• IP address + prefix length

e.g. 192.168.0.3/22

• IP header format:– Version (4 bits): IPv4, IPv6– Header length (4 bits): in 32-bit words, min

header is 5 words or 20 bytes– Type of service (8 bits): reliability, precedence,

delay, and throughput. Used in DiffServ (IP QoS support)

– Total length (16 bits): header + data in bytes, total must less than 64 KB.

– Identifier (16 bits): uniquely identify the datagram during its life for a given source and destination addess.

• IP header format:– Flags (3 bits): more flag, no-fragmentation

– Fragment offset (13 bits): in units of 8 bytes

– Time to live (TTL): specified in router hops

– Protocol: next level protocol to receive the data

– Header checksum: 1’s complement sum of all 16 bit words in the header

– Source/destination address: original source/destination

– Options (variable): Security, source route, record route, timestamp recording

– Padding (variable): makes header length a multiple of 4

– Data (variable): data + header <= 65535 bytes.

• IP functionality:– Framentation– Addressing– Routing is done by other protocols

• IP router– Routing: computes the next hop for each

destination.– Forwarding: based on the destination address in

an IP packet, forward the packet to the right next hop

• Longest prefix matching

• Internet Routing protocols– RIP: distance vector algorithm– OSPF: link state algorithm– BGP: path vector algorithm that supports policy

based routing

TCP segment structuresource port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

• TCP header– Source port (16 bits): identifies the source

process. 20=FTP, 23=Telnet, 80=HTTP.– Destination port (16 bits)– Sequence Number (32 bits): Sequence number

of the first byte in the segment. If SYN is present, this is the initial sequence number (ISN) and the first data byte is ISN+1

– ACK number (32 bits): Next byte expected– Data offset (4 bits): number of 32-bit words in

header– Reserved (6 bits)

• TCP header– Control (6 bits):

• URG urgent pointer field significant

• ACK: ACK field significant

• PSH: push function

• RST: reset the connection

• SYN: synchronize the sequence number

• FIN: no more data from sender

– Window (16 bits): wil accept [ack] to [ack+window]

– Checksum (16 bits): cover the header plus a pseudo header.

• TCP header– Urgent pointer (16 bits): points to the byte

following urgent data.– Options (variable): max segment size, window

scale factor, etc.

• Each computer has a name resolver routine (gethostbyname in UNIX)

• Each resolver knows the name of a local DNS server

• Resolver sends a DNS request to the server• DNS server either gives the answer,

forwards the request to another server, or gives a referral– Recursive query / iterative query

• Referral = next server to whom the request should be sent