Application Layer#

Application communication pattern: peer to peer or client-server

Application: software with two or more processes running in end systems that send messages over a network to accomplish some goal

Network applications use socket interface to access transport layer services

Application layer protocols are used by applications to communicate with each other. Below that, it is two hosts are communicating.

HTTP (Web)#

Evolution: HTTP -> HTTP1.1 -> HTTP/2 -> HTTP/3

HTTP request message: request line(method, URL, protocol version); request headers; request body(optional data)

HTTP response message: status line(protocol version, status code, status phrase); response header; body

HTTP performance: RTT(round trip time)

Web caches(proxy servers): deployed by ISPs

CDN: plays a imporant role in moving content closer to users in Web and streaming video services

MPEG-DASH: Dynamic Adaptive Streaming over HTTP; Client-side buffering and playout delay, and videos are divided into multiple chunks and encoded at different rates, so if poor network, CDN can provide low rates(poor quality) to maintain smooth playback without stalling.

Other Application Protocols#

SMTP(E-mail): user agents <-> mail servers

VoIP: Call on internet

SIP: session initiation protocol

RTP: real-time protocol

RTCP: real-time control protocol

DNS#

IP address <-> hostnames

13 root name servers

DNS Hierarchical: root -> top level domain -> authoritative

Summary#

  1. application(chrome, safari), they both use HTTP!
  2. application layer protocol: HTTP
  3. Application is a software with two or more processes running in the end systems to send message over a network to accomplish some goal.
  4. socket API.

Transport Layer (host to host)#

Multiplexing/demultiplexing in transport layer: receiver will demultiplex message up to different applications via socket. It is based on port number. UDP uses destination port number only while TCP uses 4-tuple: source and destination IP addresses, and port numbers.

Reliable data transfer: sender sends data and receiver waits for data.

Goal of transport layer: build reliable channel(protocol) over unreliable channel(in reality).

UDP Basis (unordered, unreliable)#

UDP datagram format: source and dest port, header length, checksum for error detection, and pay load.

UDP error detection: 16bits checksum.

TCP (segments, in-order byte stream) Basis#

Stop and wait: if Ack, move to next packet; if nAck, resend.

Corrupted Ack problem: add sequence number. If sequence number is not correct, receiver could discard duplicate packet safely.

Loss: set timeout.

Reliable data transfer mechanism: checksum, ACKs/NAKs, Retransimission, sequence number, timeouts.

It works but not efficient.

Pipelined reliable data transfer: just like water, occupied entire pipe.

Sliding window: sender buffer sent but not-yet-acknowledged packets. The window also contains not-yet-sent packets, and if window is filled with not-yet-acknowledged packets, the sender cannot send any packets.

Go-Back-N: when timeout occurs, sender will resend all not-yet-acknowledged packets(up to N).

Selective repeat(more like real world): receiver individually acknowledges each correctly received packet and buffer out-of-order packets.

TCP segment structure: source and dest port, checksum, sequence number, acknowledgment, advertised window, header length

TCP Timeout#

How to set timeout: too long: slow reaction to loss; too short: waste bandwidth. Should be close to RTT.

How to estimate RTT? dynamic RTT estimation: estimateRTT = (1 - a)estimateRTT + a * sampleRTT; estimateRTT is a safety margin, sampleRTT is the current measurement(the duration that sender receives ACK)

Fast retransmit: TCP will retransmit a segment upon receiving 3 duplicates ACKs for that segment.

TCP 3-way handshake: establish connection.

Advanced TCP#

Flow control: advertised window(RWND) tells sender how many bytes it can accept.

Congestion control:

  • congestion control: stay left of cliff
  • congestion avoidance: stay around or left of knee

Congestion window: CWND << RWND; sending rate approximates window/RTT.

Basic idea: if no congestion, increases sending rate; if timeout or 3 duplicates ACKs, decrease sending rate.

Increasing CWND:

  • Slow start: slow start for safety but ramp up quickly for efficiency(slow start threshold).
  • Congestion avoidance: move to additive increase.

Reducing CWND:

  • timeout: cut CWND to 1 MSS(max segment size), go to slow start
  • 3 duplicate ACKs: cut CWND in half, go to congestion avoidance

Summary#

  1. contorl.
  2. reliability.

Network Layer (routing, best efforts host-to-host delivery)#

  1. path-selection algorithms -> forwarding table
  2. IP protocol
  3. ICMP protocol

IP packets: TTL, IP version, checksum, header length, upper layer protocol(TCP or UDP), source and dest IP address

Data Plane vs Control Plane#

Data plane: what is the output port for the upcoming datagram at current router.

Control plane: how datagram routes among routers from src to dest.

  1. traditional approach: implemented on routers.
  2. software-defined network: implemented in remote servers/controllers.

Router Architecture#

  1. control plane: software
  2. data plane: hardware

physical layer -> datalink -> forwarding queueing -> switching fabric & routing processor -> queueing -> datalink -> physical layer

Routers:

  1. backbone routers
  2. enterprise routers
  3. access routers

Forwarding tables: based on IP dest address, use longest prefix match to find the out link interface.

Switching fabric: transfer packet from input link to appropriate outputlink

Managing queueing: too much buffering will increase delay

  1. which packet to drop when buffer is full？drop tail; priority
  2. which packet to send next from the queue？FIFO;priority scheduling

Network neutrality

Subnets and Addressing#

Subnets: IP addresses: subnets part(high common bits) and host part(low order bits).

Subnet mask: 223.1.1/24.

CIDR: Classless InterDomain Routing. address format: a.b.c.d/x, where x is the subnet portion of address.

How does host get IP address?

  1. hardcoded by system admin(static IP address - e.g. server)
  2. DHCP: dynamic host configuration protocol

DHCP(on top of UDP):

Goal: when host joins server, it automatically assigned with a IP address.

  1. host broadcasts DHCP discover msg.
  2. DHCP server offers the IP address.
  3. host request: I would use this IP address.
  4. DHCP: ok, you got this address.

NAT: network address translation, implemented in NAT router

all devices in local private network just share one public IPv4 address

Transition from IPv4 to IPv6: long time for deployment

Tunneling: IPv6 datagram carried as payload in IPv4 datagram.

Middleboxes: NAT, firewalls, load balances, caches(something taht works on data path from src host to dest host but other than router)

Routing Algorithms#

Routing algorithms classification: 1.dynamic/static; 2.global/local

  1. link state algorithm(dijkstra, static, global)
  2. distance vector algorithm(bellman-ford, dynamic, static)

Autonomous Systems (AS): aggregate routers into regions known as "autonomous systems" (AS), AS is a bunch of routers.

Interconnected ASes(real world):

  • intra-AS(OSFP, based on LS): routing among routers within same AS.
  • inter-AS(BGP, based on DV): inter-domain routing

BGP: achieving policy via advertisements

  • policy to enforce: if x does not want to route from B to C via x, so x will not advertise to B a route to C

ICMP: internet control message protocol(ping, traceroute)

Service Level Agreements#

Service Level Agreements: legal agreement between customer and network operator, defines KPI(key performance indicators)

Verizon SLAs example:

  • mean time to repair
  • availability
  • Jitter
  • latency
  • packet loss

How network operators meet these SLAs?

  • routing alogrithm: policies
  • buffer management: congestion detection
  • queue scheduling: prioritization
  • fault detection
  • virtualization: QoS(quality of service) differentiation and traffic separation

Virtual Network#

Virtual network: like virtual machine sharing the same host resource.

The advantanges of Virtual network:

  • cost
  • security
  • flexibility

Three types of VN technology:

  • VLAN
  • MPLS(multi-protocol label switching)
  • SDN (data center)

MPLS: different QoS to different service classes

LSR: label swithing router

FEC: forwarding equivalence class(a subnet of packets that are treated in the same way by LSR)

LSP: label-switched path

How it works? MPLS header encoded with FEC is inserted to a packet before packet is forwareded. Inner label tells you who you are, outer label tells you where to go.

Data center ->(SDN as control plane instead of implementing control plane inside the every single routers)

Network Management#

Network management:

  • fault
  • configuration
  • accouting
  • performance
  • security

SDN:

  • control plane: software(network apps + network OS)
  • data plane: hardware(switches)

Generalized forwarding: match + action

OpenFlow: controller-to-switch messages

Network management components:

  • SNMP（simple network management protocol
  • CLI: command line interface
  • NETCONFIG/YANG:

Summary#

  1. routing(BGP, OSPF).
  2. Virtual network(MPLS, provide different QoS).
  3. data center network(network management, SDN).
  4. SLAs(network KPIs).

Links: wired, wireless

Hosts, routers: nodes

Frame: encapsulates datagram

Analogy: you may take plane, train, taxi to your destination.

Services#

  • framing
  • reliable delivery(error control, ARQ): seldom used on wired links, wireless links have high error rate
  • half-duplex/full-duplex
  • error detection
  • error correction
  • flow control

NIC(network interface card): a chip

Error control:

  1. error detection + automatic repeat request(ARQ)
  2. forward error correction(FEC)

Error control coding(add Redundancy): parity checking, checksum

Multiple Access Channel (MAC) Protocol#

Three broad classes:

  • partitioning(TDMA, FDMA): inactive channel, waste
  • random access(ALOHA, slotted ALOHA): allow collisions
  • taking turns(polling)

Slotted ALOHA:

Assumptions: same size frames, time divided into equal size slots, and nodes are synchronized

if collision, retransmit frame with probability p(randomization).

CSMA(carrier sense multiple access):

if channel sensed idle, transmit.

if channel sensed busy, defer transimition.

CSMA/CD(generic idea): CSMA withcollision detection

Ethernet CSMA/CD algorithm: implementation, binary exponential backoff

Taking turns protocols:

  1. polling(bluetooth): centralized controller invites other nodes to transmit
  2. token passing(token ring): pass control token message to the next one

MAC Address and ARP#

MAC address

ARP(address resolution protocol): find MAC address for certain IP

  1. addressing within subnet: broadcast and target MAC replies
  2. routing to another subnet: link layer <-> network layer

Ethernet frame: MAC src and dest, and data and CRC.

Ethernet(wired LAN) bus -> switched: Ethernet protocol, switch, MAC address

Switch: forwarding, link layer device, self-learning(broadcast)

MST: Sometimes we add addtional link to provide redundant connections to prevent potential problems, this loop could lead to infinite loop(no TTL in frame), so we need use MST to prevent this.

Institutional netowrk: switched LANs, Ethernet, MST

VLAN implementation:

  1. port based
  2. MAC address based(defined MAC list at switch)
  3. high layer based(protocol based)

Bridged LANs(IEEE 802.1Q):

  1. VLAN tags
  2. flow control
  3. priority control

Summary#

  1. node, Link.
  2. error detection, correction; multiple access(sharing a broadcast channel); link layer addressing(ARP)
  3. institutional network: MST, Switched LANs, Ethernet
  4. ethernet frame

Physical Layer(signals and media)#

signals + encode/decode + media

wireless e2e: source(text) -> source encoder(bits) -> channel encoder(digital signals) -> modulator(analog signal) -> channel -> demodulator -> channel decoder -> source decoder -> source

  • guided media: copper wire(twisted pair, coax): good conductor of electricity and abundant optical fiber: internal reflection, glass is cheap and made of sand, environmental friendly.
  • unguided media: Antennas: electromagnetic radiation

physical medium:

  • wire pair
  • coaxial cable
  • optical fiber

wireless examples:

  • broadcast(one-way): radio, TV
  • two-way: bluetooth, cell phone

sin(math): frequency, period, phase electromagnetic waves(physics): frequency, period, phase, wavelength

decibels: power of signals, because of the range of signal power is so broad, so we use base 10 logarithm to quantify values.

modulation(wireless):

  • baseband
  • carrier frequency

SNR: signal to noise rate shannon capacity bound maxmimum channel capacity: depends on bandwidth and SNR

Link budget: Path loss = Pt - Pr + Gains - Margins wireless Link budget: free space model

optical Links: WDM

Other Topics#

Network performance:

  • delay
  • loss
  • throughput

Data center network

  1. outward facing: north-south traffic.
  2. computations: east-west traffic. Components and costs:
  3. servers(CPU, memory, disk): 45%.
  4. power infra(cooling).
  5. power(electrical costs).
  6. network(links, switches). multi-vms in a physical machine: vSwitch to manage the traffic data center network protocol innovation AIOPs: AI for network operations

Wireless network Typically used in access network(first hop is wireless)

topologies:

  1. infra based: wireless to wired network
  2. ad hoc based: single hop, family network
  3. multi-hop: military communication network

elements of wireless network:

  1. base station
  2. infra mode(wired network, handoff(mobile change stations))
  3. wireless link(mobile)

wireless chacacteristic:

  1. multipath
  2. noise
  3. mobility
  4. hidden terminal

coverage:

  1. Interference to/from other transmitters
  2. number of access points that are required to provide service in a space

Link budget: theorotical analysis

adaptive modulation and coding(AMC): maximum data rates and quality

WLAN 802.11: different link layer and physical layer protocol stack structure

elements of WLAN:

  1. access point(AP)
  2. basic service set(BSS)
  3. basic service area(BSA, cell)

WLAN topology: star, AP is the hub

WLAN protocol stack: different from wired network

WLAN mechanism:

  1. IFS: DIFS, SIFS(ACK)
  2. RTS/CTS(hidden terminal)

different frames

beacon(association for new devices)

handoff(passive and active scanning)

power management

WLAN security

Cellular network: 4g and 5g

sleep mode: mobile device has sleep mode to save energy

multi-access: OFDMA, time and freqency multiplexing

resource blocks: based on OFDMA

WPAN wireless personalized area network: bluetooth

Key Concepts#

  1. CDN(content delivery netowrk): caching for speed and replica for backup.
  2. Data plane: forwarding, fast, based on hardware.
  3. Control plane: routing, slow, based on software.
  4. AS: autonomous systems. Tier 1 AS: global ISP -> Tier 2 AS: regional ISP -> Tier 3 AS: local ISP(like Pitt)
  5. Delay Jitter: variation in delay. Jitter buffer.
  6. Web pages: base HTML file and several referenced objects. Objects are identified by URL(unique resource locater).