exam

Chapter 3

UDP

  • Checksum calculation

Binary addition of the source port + dest. port + length checksum.png

Carrying the one will wrap around to the beginning.

A handy python script I came up with: 

# calculate checksum
def checksum(numbers, hex=False):

    if len(numbers) > 3:
        print("this doesn't seem like udp")
    length = len(numbers[0])
    if hex:
        wrap = int("f"*length, 16)
    else:
        wrap = int("1" * length, 2)
    result = 0
    for i in numbers:
        if hex:
            i = int(i, 16)
        else:
            i = int(i, 2)
        result += i
        if result > wrap:
            result -= wrap
    result = bin(result ^ wrap)[2:]
    result = (length-len(result)) * "0" + result
    return result
if __name__ == "__main__":
    print(checksum(["0110011001100000", "0101010101010101", "1000111100001100"]))

\includepdf[pages=205-211, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

TCP

  • LastByteRcvd - LastByteRead <=RcvBuffer
  • rwnd = RcvBuffer - [LastByteRcvd - LastByteRead]

fast retransmit

  • If sender receives 3 duplicate acks, segment has been lost, segment is resent before timer expires

flow control

  • receive window is used to give the sender an idea of ho much free buffer space is available.
  • UDP does not have flow control

three way handshake

  • client sends an initial segment (no payload), server sends an initial segment as a reply (no payload), client sends another segment back (maybe payload) 
\includepdf[pages=238-266, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

TCP congestion control

  • Slow start :Initial sending rate is MSS/RTT (Max segment size/Round trip time), increases congestion window by 1 MSS every acknowledged segment. Sending rate doubles every RTT.
  • congestion avoidance: adds 1MSS each RTT
  • Fast recovery: Congestion window is increased by 1 MSS for every duplicate ACK
  • TCP Tahoe: Cut congestion window to 1 MSS and entered slow start after either timout indicated or triple-duplicate ack loss event.
  • TCP Reno: incorporated fast recovery
  • AIMD (Additive increase Multiplicative-decrease): TCP congestion control, saw tooth shape
  • TCP throughput: average throughput of a connection = \(\frac{0.75 \cdot W}{RTT}\) 
\includepdf[pages=274-290, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

Chapter 4

IP

  • Header checksum (split each 2 bytes in the header as a number and summing them, then taking the 1s compliment)
  • IPv4: datagram.png
  • addressing: 32 bits long
  • subnetting: a subnet mask splits the IP address into two halves, the left of which is the subnet mask address.
  • DHCP: dynamic host configuration protocol
  • NAT: network address translation
  • IPv6: datagram2.png 
\includepdf[pages=341-364, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

Chapter 5

Routing Algorithms

  • link state (LS) routing - Dijkstra's algorithm
  • Distance-vector (DV) routing algorithm 
\includepdf[pages=391-406, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

Intra-AS routing in the internet: OSPF

  • open shortest path first
  • dijkstra's
  • intra-as 
\includepdf[pages=406-410, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

ROuting among the ISPs: BGP

  • inter AS protocol
  • iBGP between routers in AS
  • eBGP between routers in different AS 
\includepdf[pages=410-422, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

ICMP

  • Internet control message protocol 
\includepdf[pages=434-436, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

Chapter 6

Error-Detection and Correction techniques

  • parity bits
  • CRC 
\includepdf[pages=465-472, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}

Switched Local Area Networks

  • MAC addressing
  • ARP
  • self-learning mechanism of switches
  • switches vs routers vs hubs 
\includepdf[pages=488-512, pagecommand={}]{computer-networking-a-top-down-approach-8th-edition.pdf}