COMP211-W4

Lec 4 - Network Layer - Data Plane

网络层

Goal

  • Principles behind network layer services, focusing on the data plane:
    • Network layer service routing
    • how a router works
    • addressing
    • generalized forwarding
    • Internet architecture
  • Instantiation, implementation in the Internet
    • IP protocol
    • NAT
    • Routing protocols

控制平面:全局
数据平面:局部

Overview:

  • Data Plane
  • Control Plane

Network-layer services and protocols

  • transport segment from sending to receiving host
    • sender: encapsulates segments into datagrams, passes tp link layer
    • receiver: deliver segments to transport layer protocol
  • network layer protocols in every Internet decvice: hosts, routers
  • routers:
    • examines header fields in all IP datagrams passing through it
    • moves datagrams from input ports to output ports to transfer datagrams along end-end path

Two key network-layer functions

network-layer functions:

  • forwarding: move packets from a router’s input link to appropriate router output link
  • router: determine route taken by oackets from source to destination
    • routing algorithms

analogy: taking a trip

  • forwarding: process of getting through single interchange
  • routing: process of planning trip from source to destination

Network layer: data plane, control plane

Data plane:

  • local, per-router function
  • determines how datagram arriving on router input port is forwarded to router output port

Control plane

  • network-wide logic
  • determines how datagram is routed among routers along end-end path from source host to destination host
  • two control-plane approaches:
    • traditional routing algorithms: implemented in routers
    • software-defined networking(SDN): implemented in (remote) servers

Per-router control plane

Individual routing algorithm components in each and every router interact in the control plane

SoftWare-Defined Networking(SDN)control plane

Remote controller computes, installs forwarding tables in routers

Inside a router

Router acrchiteture overview

High-level view of generic router architecture:

Input port functions

分散的切换

  • destination-based forwarding: forward based only on destination IP address(traditional)
  • generalized forwarding: forward based on any set of header field values

Destination-based forwarding

Longest prefix matching

Longest prefix match

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

Example 1: interface:0
Example 2: interface:1

Switching fabrics

  • Transfer packet from input link to appropriate output link

  • switching rate: rate at which packets can be transfer from inputs to outputs

    • often measured as multiple of input/output line rate

    • N inputs: switching rate N times line rate desirable

    • three major types of switching fabrics:

Switching via memory

First generation routers:

  • traditional computers with switching under direct control of CPU
  • packet copied to system’s memory
  • speed limited by memory bandwidth(2 bus crossing per data gatagram)

Switching via a bus

  • datagram from input port memory to output port memory via a shared bus
  • bus connection: switching speed limited by bus bandwidth
  • 32 Gbps bps, Cisco 5600: sufficient speed for access routers

Switching via interconnection network

  • Crossbar, Clos networks, other interconnection nets initially deceloped to connecr processors in multiprocessor
  • multistage switch: nxn switch from miltiple stages of smaller switches
  • exploiting parallelism:
    • fragment datagram into fixed length cells on entry
    • switch cells through the fabric, reassemble datagram at exit

Input port queuing

  • If switch fabric slower than input ports combined -> queueing may occur at input queues
    • queueing delay and loss due to input buffer overflow!
  • Head-of-the-Line(HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

Output port queuing

  • Buffering required when datagrams arrive from fabric faster than link transmission rate.

Drop policy: which datagrams to drop if no free buffers?

Datagrams can be lost due to congestion, lack of buffers

  • Scheduling discipline chooses among queued datagrams for transmission

Priority scheduling - who gets best performance, network neutrality

IP: the Internet Protocol - Datagram format

Network Layer: Internet

host, router network layer functions:

IP Datagram format

IP fragnebtation/reassembly

  • network links have MTU(max, transfer size) - largest possible link-level frame
    • different link types, different MTUs
  • large IP datagram divided(“fragmented”) within net
    • one datagram becomes several datagrams
    • “reassemebled” only at destination
    • IP header bits used to identify, order related fragments

      IP fragmentation/reassembly

IP: the Internet Protocol - Addressing

IP addressing: introduction

  • IP address: 32-bit identifier associated with each host or router interface
  • interface: connection between host/router and physical link
    • router’s typically have mulriple interfaces
    • host typically has one or two interfaces(e.g., wired Ethernet, wireless 802.11)

Subnets


Recipe for defining subnets:

  • detach each interface from its host or router, creating “islands” of isolated networks
  • each isolated network is called a subnet

IP addressing: CIDR

CIDR: Classless Inter Domain Routing(pronounced “cider”)

  • subnet protion of address of arbitrary length
  • address format: a.b.c.d/x, where x is # bits subnet portion of address

IP: the Internet Protocol - get an IP Address

IP addresses: how to get one?

Q: How does a hostget IP address within its network (host part of address)?

Q: How does a networkget IP address for itself (network part of address)

How does hostget IP address?

  • hard-coded by sysadmin in config file (e.g., /etc/rc.config in UNIX)
  • DHCP:Dynamic Host Configuration Protocol: dynamically get address from as server•“plug-and-play”

DHCP: Dynamic Host Configuration Protocol

goal: host dynamically obtain IP address from network server when it “joins” network

  • can renew its lease on address in use
  • allows reuse of addresses (only hold address while connected/on)
  • support for mobile users who join/leave network

DHCP overview

  • host broadcasts DHCP discovermsg [optional]
  • DHCP server responds with DHCP offermsg [optional]
  • host requests IP address: DHCP request msg
  • DHCP server sends address: DHCP ackmsg

DHCP client-server scenario


DHCP: more than IP addresses

DHCP can return more than just allocated IP address on subnet:

  • address of first-hop router for client
  • name and IP address of DNS server
  • network mask(indicating network versus host portion of address)

IP addresses: how to get one

Hierarchical addressing: route aggregation

hierarchical addressing allows efficient adverrisement of routing information:


IP addressing: last words

IP: the Internet Prorocol - Network address translation & IPv6

NAT: network address translation

NAT: all devices in local network share just one IPv4 address as far as outside world is concerned

IPv6: motivation

  • initial motivation: 32-bit IPv4 address space would be completely allocated
  • additional motivation:
    • speed processing/forwarding: 40-byte fixed length header
    • enable different network-layer treatment of “flows”

IPv6 datagram format

Transition from IPv4 to IPv6

  • not all routers can be upgraded simultaneously
    • no “flag days”
    • how will network operate with mixed IPv4 and IPv6 routers
  • tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers(“packet within a packet”)
    • tunneling used extensively in other contexts(4G/5G)

Tunneling and encapsulation

Tunneling

IPv6: adoption

  • Long(long!) time for deployment, use
    • 25 years and counting
    • think of application-level changes in last 25 years: WWW, social media, streaming media, gaming, telepresence,….

Generalized Forwarding, SDN - Match + action & OpenFlow

Generalized forwarding: match plus action

Review: each router contains a forwarding table (aka: flow table)

  • “match plus action” abstraction: match bits in arriving packet, take acton
    • destination-based forwarding” forward based on dest. IP address
    • generalized forwarding:
      • many header fields can determine action
      • many action possible: drop/copy/modify/log packet

Flow table abstraction

  • flow: defined by 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 matched packet, or send matched packet to controller
    • priority: disambiguate overlapping patterns
    • counters: #bytes and #packets

OpenFlow: flow table entries

  • example

OpenFlow abstraction

  • match + action: abstraction unifies different kinds of devices

Router

  • match: longest destination IP prefix
  • action: forward out a link

Switch

  • match: destination MAC address
  • action: forward or flood

Firewall

  • match: IP address and TCP/UDP port numbers
  • action: permit or deny

NAT

  • match:IP address and port
  • action: rewrite address and port

Example

Generalized forwarding: summary

Lec5 - Network Layer - Control Plane

Goals:

  • Principles behind network control plane:
    • tradtional routing algorithms
    • SDN controllers
  • Instantiation, implementation in the Internet:
    • OSPF,BGP
    • OpenFlow

Introduction

Network-layer functions

  • forwarding(转发): move packets from router’s input to appropriate router output
  • routing(路由): determine route taken by packet from source to destination

Per-router control plane

Individual routing algorithm components in each and every
router interact in the control plane

Software-Defined Networking(SDN) control plane

Remote controller computes, installs forwarding tables in routers

Routing Algorithms

Routing protocols

Routing algorithm classification

Intra-AS Routing in the Internet: OSPF

Making routing scalable

Inrernet approach to scalable routing

aggregate routers into regions known as “autonomous systems” (AS) (a.k.a. “domains”)

inter-AS(aka “inter-domain”): routing among AS’es

  • gateways perform inter-domain routing(as well as intra-domain routing)

intra-AS(aka “intra-domain”): routing within same AS(“network”)

  • all routers in AS must run same intra-domain protocol
  • routers in different AS can run different intra-domain routing protocols
  • gateway router: at “edge” of its own AS, has link(s) to router(s) in other AS’ es

Interconnected ASes

Inter-AS routing: routing within an AS

most common intra-AS routing protocols:

  • RIP:Routing Information Protocol
    • classic DV: DVs exchanged every 30 secs
    • no longer widely used
  • EIGRP: Enhanced Interior Gateway Routing Protocol
    • DV based
    • formerly Cisco-proprietary for decades (became open in 2013)
  • OSPF: Open Shortest Path First
    • link-state routing
    • IS-IS protocol (ISO standard, not RFC standard) essentially same as OSPF

OSPF(Open Shortest Path First) routing

  • “open”: publiciy available
  • classic link-state
    • each router floods OSPF link-state advertisements (directly over IP rather than using TCP/UDP) to all other routers in entire AS
    • multiple link costs metrics possible: bandwidth, delay
    • each router has full topology, uses Dijkstra’s algorithm to compute forwarding table
  • secruity: all OSPF messages authenticated(to prevent maliciious intrusion(恶意入侵))

Hierarchical OSPF

Routing Among the ISPs: BGP

Internet inter-AS routing: BGP

  • BGP(Border Gateway Protocol): the de facto inter-domain routing protocol
    • “glue that holds the Internet together”
  • allows subnet to advertise its existence, and the destinations it can reach, to rest of Internet: “I am here, here is who I can reach, and how”
  • GBP provides each AS a means to:
    • eBGP: obtain subnet reachability information from neighboring ASes
    • iBGP: propagate reachability information to all AS-internal routers.
    • determine “good” routes to other networks based on reachability information and policy

eBGP, iBGP connections

BGP basics

  • BGP session: two BGP router(“peers”) exchange BGP messages over semi-perimanent TCP connection:
    • advertising path to different destination network prefixes(BGP is a “path vector” protocol)
  • when AS3 gateway 3a advertises path AS3, x to AS2 gateway 2c:
    • AS3 promises to AS2 it will forward datagrams towards X

Path attributes and BGP routes

  • BGP adverised router: prefix + attributes
    • prefix: destination being advertised
    • two important attributes:
      • AS-PATH: list of ASes through which prefix advertisement has passed
      • NEXT-HOP: indicates specific internal-AS router to next-hop AS
  • policy-based routing:
    • gateway receiving route advertisement uses import policy to accept/decline path (e.g., never route through AS Y).
    • AS policy also determines whether to advertise path to other other neighboring ASes

BGP path advertisement




BGP route selection

  • router may learn about more than one route to destination AS, selects route based on:
    1. local preference value attribute: policy decision
    2. shortest AS-PATH
    3. closest NEXT-HOP router: hot potato routing
    4. additional criteria
  • RouteViews Project: http://www.routeviews.org
    • telent route-views.linx.routeviews.org
    • show ip bgp 130.127.0.0/16 longer-prefixes

Why different Intra-, inter-AS routing?

policy:

  • inter-AS: admin wants control over how its traffic routed, who routes through its network
  • intra-AS: single admin, so policy less of an issue

scale:

  • hierachical routing saves table size, reduced update traffic

performance:

  • intra-AS: can focus on performance
  • inter-AS: policy dominates over performance

Software defined networking(SDN)

  • Internet network layer: historically implemented via distributed, per-router control approach:
    • monolithic router contains switching hardware, rums proprietary implementation of Internet standard protocols (IP, RIP, IS-IS, OSPF, BGP) in proprietary router OS(e.g., Cisco IOS)
    • different “middleboxes” for different network layer functions: firewalls, load balancers, NAT boxes, …

Software-Defined Networking(SDN) control plane

Remote controller computes, installs forwarding tables in routers

Software defined networking(SDN)

SDN analogy: mainframe to PC revolution

Traffic engineering: difficult with traditional routing



The SDN Control Plane - Implementation

Software defined networking(SDN)



OpenFlow protocol

  • operates between controller, switch
  • TCP used to exchange messages
    • optional encryption
  • three classes of OpenFlow messages:
    • controller-to-switch
    • asynchronous(switch to controller)
    • symmetric(e.g. check liveness)

SDN: control/data plane interaction example


SDN and the future of traditional network protocols

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很惭愧<br><br>🌳就是最菜的<br>谢谢大家