Computer Networks midterm exam notes

Computer Networks notes for use during the midterm exam (I will supply you with a copy with your exam)

Datagram forwarding: each switch extracts the destination address from the packet, looks that address up in its ⟨dest,next_hop⟩ forwarding table, and then forwards the packet to the matching next_hop neighbor.

Virtual-circuit forwarding: each switch notes the arrival interface (port), and then looks up the ⟨VCI_in, port_in⟩ in its connections table, retrieving ⟨VCI_out, port_out⟩. The VCI field is then rewritten to VCI_out, and the packet is forwarded via port_out.

Knee congestion: queue has started to build. Cliff congestion: packets are dropped.

Switched Ethernet: a switch learns the port by which to reach destination A when a packet from A arrives.

NRZ: transition on 1-bits, no transition on 0-bits. 4B/5B: each 4-bit data pattern is transmitted as a 5-bit pattern with "enough" 1-bits, using NRZ.

IP addresses: The length of the network prefix is often denoted with a slash, eg 147.126.0.0/16. The slash notation works also with subnets, and corresponds to the subnet mask.

Subnets involve moving the net/host division point further right. This allows hierarchical routing within an organization, while at the same time allowing use of a single network prefix to reach the organization.

Bandwidth delay = packet_size / bandwidth. Propagation delay = time for one bit to travel from source to destination.

Store-and-forward delay: if a packet passes through a router/switch R, then R must read the entire packet before beginning to retransmit it. This adds one more bandwidth delay.

Either the sender or the receiver (or both) can implement retransmit-on-timeout; usually it is the sender. The other side then implements retransmit-on-duplicate. The Sorcerer's Apprentice bug may result if both sides implement retransmit-on-duplicate.

The bottleneck link is the link with the smallest bandwidth.

Sliding windows formulas:

optimal winsize = bottleneck_bandwidth * RTT_noLoad
queue_time = RTT_actual − RTT_noLoad
throughput = winsize/RTT_actual
queue_usage = throughput × (RTT_actual − RTT_noLoad)
RTT_actual = winsize/bottleneck_bandwidth
queue_usage = winsize − bandwidth × RTT_noLoad
RTT_actual/RTT_noLoad = winsize/transit_capacity = (transit_capacity + queue_usage) / transit_capacity

If host A knows B's IP address, has determined it is on the same LAN, and wants to find B's physical address, it broadcasts an ARP query. B receives this, and responds directly to A. The pair ⟨B_IP,B_phys⟩ is then entered into A's ARP cache.

Routers using distance-vector algorithms exchange tables of ⟨dest,cost⟩ pairs. If router A receives ⟨D,c_D⟩ from neighbor N at distance c_N, A updates its table as follows:

D is a previously unknown destination: A adds ⟨D,N,c_D+c_N⟩
D is a known destination with entry ⟨D,M,c⟩, but c > c_D+c_N: A updates the entry for D to ⟨D,N,c_D+c_N⟩.
A has a previous entry ⟨D,N,c⟩, but c < c_D+c_N: A updates the entry for D to ⟨D,N,c_D+c_N⟩.

Split horizon: N does not report a destination to D if N uses D as its next_hop for that destination

Triggered updates: If N discovers a problem, it sends out a new report immediately

Canonical transport issues:

old duplicate packets
lost final ACK
duplicated connection request
reboots

TFTP:

Receiver sends initial RRQ (Read ReQuest) packet
Sender replies from a new port; receiver "latches on" to that new port
Stop-and-wait is used
The response to a data packet from a bad port is an error packet

TCP only allows data transmission after a connection has been established. Connection establishment: three-way handshake (3WHS):

    ---> SYN -->
    <--SYN+ACK
    ---> ACK -->

Each side includes its Initial Sequence Number, or ISN, in its SYN.

FIN closes the connection. When one side sends a FIN, it in effect promises not to send more data, though the other side may still send data.