Chapter 4: Network layer(Under Construction)

Network layer overview

• transport segment from sending to receiving host
• sending: encapsulates segments into datagrams
• receiving: delivers segments to transport layer
• network layer protocols in every host and router
• router examines header field in all IP datagrams passing through it

Functions of network-layer

• forwarding: move packets from router to another appropriate router
• routing: determine packet route from source(下稱src) to destination(下稱dst)
• 類比：forwarding像轉程點，routing像是路線規劃。

Data and control plane

Data plane

• local, per-router function
• determine how datagram arriving on router input port and forwarding(轉發) to output port
• forwarding function

Control plane

• network-wide logic
• determines how datagram is routed among routers from src and dst
• two approaches: traditional routing implemented in routers, softward-defined networking(SDN): implemented in (remote) servers

Per-router control plane

• Individual routing algorithm in each router interact in the control plane

Logically centralized control plane

• A distinct (typically remote) controller interacts with local control agents(CAs)

Network service model

• What service model for "channel" transporting datagrams from sender to recevier?
• example services for individual datagrams: guarateed delivery with no or certain time
• example services for a flow of datagrams: in-order datagram delivery, guaranteed minimum bandwidth to flow, restrictions on changes in inter-packet spacing.
• ATM stands for Asynchronous Transfer Mode here, NOT Automatic teller machine!!
• ATM網路架構的四種模式
  

Router arch. overview

• high-level view of generic router arch:
  

Input port functions

• physical layer(Green rectangle): bit-level reception
• data link layer(Blue rectangle): e.g. Ethernet(Chapter 5)

Decentralize switching(Red rectangle):

• using header field value, lookup output port using forwarding table in input port memory("match plus action")
• goal: complete input port processing at "line speed"
• queuing: if datagrams arrive rate > forwarding rate
• dst-based forwarding: forward based only on dst IP addr
• generalized forwarding: forward based on any set of header field value

Longest prefix matching

• longest prefix matching is used shortly
• often performed using ternary(三個一組的) content addressable memories(TCAMs)
• content addressable: present address to TCAM: retrieve address in one clocy cycle, regardless of table size
• Cisco Catalyst: can up ~1M routing table entries in TCAM

Switching fabries

• transfer packet from input buffer to appropriate output buffer
• switching rate: rate at which packets can be transfer from inputs to outputs
• 3 types of switching fabrics

tags: Computer Network CSnote