Overview

Application Layer

HTTP: on TCP or other
SMTP
DNS
FTP

Transport Layer

RDT
GBN
Selective Repeat
Congestion Control
Connection Establish and Release
- SYN 1 1 0
- FIN 1 1

Network Layer

IP CIDR
DHCP NAT
Routing Algorithm
OSPF BGP

Link Layer

CRC checksum

Protocol Layer

Application Layer: support network applications (FTP, HTTP, SMTP)
Transport Layer: host-host data transfer (TCP, UDP)
Network Layer: Routing of Datagrams from source to destination. (IP, Routing protocol)
Link Layer: data transfer between neighboring network elements.
Physic Layer: bits on the wire.

Application Layer

HTTP

Runs on TCP(most). Non-persistent HTTP or persistent HTTP
use of cookies
200OK, 301move permanently, 400bad request, 404not found, 505version not supported

SMTP: for email

Telnet

like SSH, but not as safe as SSH(encrypted transmission).

FTP

transfer files between two different system.

DNS

Runs on UDP
Root name sever --> TLD sever --> Authoritative server (local DNS server)

Transport Layer⚠️

UDP

no connection
unreliable transformation
Neck: source port, destination port, data length

TCP

Neck: source port, destination port, SEQ number, ACK number, ACK, SYN, FIN, window

RDT

aka reliable data transfer

RDT1.0: lower layer are reliable

RDT2.0: lower layer may make mistakes on bits.

ACK&NAK, auto-resend.
checksum
waiting

RDT2.1: based on RDT2.0

SEQ number
checksum in ACK/NAK

RDT2.2: improve RDT2.1

no NAK (NAK-free)
ACK the last pack
sender recv repeated ACK, which means NAK

RDT3.0: lower layer may lost data pack or mistake on bits.

timer
pipeline
- Go Back N (GBN)
- Selective Repeat (SR)

TCP Connection

Establish

client send (SYN=1, SEQ=x) to server.
server send (ACK=x+1, SYN=1, SEQ=y) to client.
client send (ACK=y+1, SEQ=x+1) to server.
connection established!

Release

client send (FIN=1, SEQ=u) to server.
server send (ACK=u+1, SEQ=u) to client. then server wait for finishing remained data transfer.
server send (FIN=1, ACK=u+1, SEQ=w) to client.
client send(ACK=w+1, SEQ=u+1) to server.
connection released!

Network Layer

IP-address: net-id(subnet mask) + host-id

CIDR

Address format: a.b.c.d/x, where x is # bits in subnet part of address.

Process

If the dest net-id in the same subnet of the router, deliver to the dest port or use routing table to find the dest port.

DHCP & NAT

Routing Algorithm

discover neighbors and learn their network address
measure the delay and cost to each of its neighbors
send a echo packet and request, the neighbors response immediately 1/2*total cost of time is the delay.
envelope their just learned knowledges into the flooding packet.
routers regularly envelope the packets
send the flooding packet to all the other routers
fundamental operation
- every node send its delay to the neighbors to the other nodes and forward the received flooding packet to all directions expect its incoming direction.
avoid to much flooding packet
- every packet have a TTL value and the value reduce 1 after each forward.
after the floods all the router have the same topology structure then use dijkstra’s algorithm

Hierarchical

OSPF aka Open Shortest Path First VS BGP Broader Gateway Protocol

OSPF: a protocol that we use in one-single AS(Autonomous System) to find the shortest path.

BGP: a protocol that we use in many ASs to find the shortest path among Ass.

iBGP(internal), eBGP(external)

Link Layer

CRC, aka cyclic redundancy check. more detail

z.B.

sender compute CRC

raw data: 1101011011, k=10

generative poly: 10011, n+1=5, n=4

raw data add n 0s: 1100101(0000)
then mod-2 divide 10011, remainder (1110) is CRC checksum.
send 1101011011(1110)

(mod-2 divide is 2 binary string XOR, 不进位(no carry)且保证第一位为 0)

receiver check CRC

1101011011(1110)
11010110111110 mod-2 divides 10011, if the remainder is 0000, checksum passed.