Network layer: Data plane¶

Service Model¶

Defines the characteristics of end-to-end delivery of packets

• Individual packet:
  • Guaranteed delivery
  • Guaranteed delivery with bounded delay
  • security
• Datagram flow:
  • in-order datagram delivery
  • guaranteed minimum bandwidth flow
  • restrictions on changes in inter-packet spacing

Best effort service model with Internet: No guarantee on anything.

Pro:

• simplicity → widely deployed/adopted
• with sufficient bandwidth, can provide real-time/band-comsuming services

Success!

Router¶

Generic architecture¶

Input port functions¶

Destination-based Forwarding¶

• based only on destination IP address(traditional)
• Multiple Matching: longest prefix matching
  • look up devices: TCAMs(Ternary content addressable memories)
    • address → content in 1 clock cycle

Switching¶

Memory	Bus	Interconnection Network

Memory: Can utilize at most half of the memory bandwidth

Bus: all output port can receive the packet; only one packet can transmitted on the bus

Multistage switch:

• build big switcher from small ones
• enable parallelism by multiple switching planes

Queuing¶

Input port: switch fabric slower than input ports combined → queuing delay & loss of packet

Head-of-the-line(HOL) blocking: queued datagram at front of the queue prevents others in queue from moving forward (even output port is free at that time)

Output port queuing: when arriving speed is faster than link transmission rate, need a scheduling discipline chooses among queued datagrams for transmission(may have priority)

Buffer¶

\(N\) independent TCP flows, buffering equals to \(B = \frac{\text{RTT}\cdot C}{\sqrt N}\)

Packet Scheduling¶

• Drop
• Marking (Congestion signal)

Packet scheduling: which queued packets are transmitted to output port

• FCFS: First-come-first-served
• Priority: FCFS with multiple priority level
• Round robin: queuing by class
• weighted fair queuing: Generalized Round robin

IP: the Internet Protocol¶

Datagram Format¶

IP address¶

32-bit identifier associated with each host or router's interfaces

"Interfaces": the boundary between the host and the physical link

• router can have multiple interfaces
• host typically have one or two interfaces
  • Wireless
  • wired ethernet

Dotted-decimal notation: 233.233.233.233

Subnet¶

the part of internet that can physically reach each other without passing a router

Detach interface from hosts/routers, create isolated subnet

CIDR: Classless Interdomain Routing¶

or subnet mask

a.b.c.d/x : leftmost \(x\) bits are network part(network prefix) of IP address

broadcast IP: 255.255.255.255, send to all host in subnet

Classful addressing: subnet can only have 8-, 16- or 24- bit subnet address, known as class A, B and C network

DHCP(Dynamic Host Configuration Protocol)¶

Goal:

• zero configuration/plug-and-play
• renew its lease on address in use
• allow reuse of addresses
• support for mobile users

client-server protocol

DHCP server or DHCP relay agent needed(router)

UDP port 67 & 68

Process (suppose that server is in subnet):

1. host broadcasts: DHCP discover msg[optional]

   1. destination: 255.255.255.255
   2. this host: 0.0.0.0

2. DHCP server responds with DHCP offer msg[optional]

   1. broadcast
   2. use transaction ID to distinguish different client
   3. have an IP address valid(lease) time

3. host request IP address: DHCP request msg

4. DHCP server sends address: DHCP ack msg

Other function of DHCP:

1. address of first-hop router for client
2. name & IP address of DNS server
3. network mask

ICANN & IP allocation¶

NAT¶

all devices in local network share just one IPv4 address to outside

leave local network with same source address, different port number

10/8, 172.16/12, 192.168/16 prefix are private networks

advantages:

• just need one public IP address
• can change local address/ISP without noticing other parts
• privacy

implementation: NAT router, maintain a NAT translation table

• assign port number for outgoing datagram

disadvantages:

• violate end-to-end arguments
• need NAT traversal to connect a server behind NAT router

IPv6¶

128-bit address

Datagram Format¶

• no checksum(end-to-end)
• no fragmentation/reassembly
• no options

Tunneling: IPv6 datagram carried as payload in IPv4 datagram

Generalized Forwarding & SDN¶

Generalized Forwarding: based on any set of header field values

abstraction: match plus action(match bits in arriving packet, take action)

Flow table¶

flow: header field values(in link-, network-, transport-layer fields)

generalized forwarding: simple packet-handling rules

• match: pattern values in packet header fields
• actions: for matched packet
  • drop
  • forward
  • modify [change counter/address]
  • send to controller
• priority
• counters: #bytes, #packets and #time

Can be used as: router, firewall, switch, NAT

Middleboxes¶

“any intermediary box performing functions apart from normal, standard functions of an IP router on the data path between a source host and destination host”

Three types of services performed by middleboxes:

• NAT Translation
• Security Services
• Performance Enhancement