数据链路层和局域网(The Link Layer and LANs)

introduction, services

Link layer: introduction

terminology:

• hosts and routers: nodes
• communication channels that connect adjacent nodes along communication path: links
• • wired links
• • wireless links
• • LANs
• layer-2 packet: frame([链路层]帧), encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link layer: context

datagram transferred by different link protocols over different links:

• e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link

each link protocol provides different services

• e.g., may or may not provide rdt over link

transportation analogy:

• trip from Princeton to Lausanne
• • limo: Princeton to JFK
• • plane: JFK to Geneva
• • train: Geneva to Lausanne
• tourist = datagram
• transport segment = communication link
• transportation mode = link layer protocol
• travel agent = routing algorithm

Link layer services

• framing, link access:
• • encapsulate datagram into frame, adding header, trailer
• • channel access if shared medium
• • “MAC” addresses used in frame headers to identify source, destination
• • • different from IP address!
• reliable delivery between adjacent nodes
• • we learned how to do this already (chapter 3)
• • seldom used on low bit-error link (fiber, some twisted pair)
• • wireless links: high error rates

Q: why both link-level and end-end reliability?

• flow control: pacing between adjacent sending and receiving nodes
• error detection:
• • errors caused by signal attenuation, noise.
• • receiver detects presence of errors:
• • • signals sender for retransmission or drops frame
• error correction:
• • receiver identifies and corrects bit error(s) without resorting to retransmission
• half-duplex and full-duplex
• • with half duplex, nodes at both ends of link can transmit, but not at same time

Where is the link layer implemented

• in each and every host
• link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
• • Ethernet card, 802.11 card; Ethernet chipset
• • implements link, physical layer
• attaches into host’s system buses
• combination of hardware, software, firmware

Adaptors communicating

• sending side:
• • encapsulates datagram in frame
• • adds error checking bits, rdt, flow control, etc.

• receiving side

• • looks for errors, rdt, flow control, etc.
• • extracts datagram, passes to upper layer at receiving side

error detection, correction

Error detection

EDC = Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!
• • protocol may miss some errors, but rarely
• • larger EDC field yields better detection and correction

Parity checking

single bit parity: detect single bit errors

two-dimensional bit parity: detect and correct single bit errors

Cyclic redundancy check

• more powerful error-detection coding
• view data bits, D, as a binary number
• choose r+1 bit pattern (generator), G
• goal: choose r CRC bits, R, such that
• • exactly divisible by G (modulo 2)
• • receiver knows G, divides by G. If non-zero remainder: error detected!
• • can detect all burst errors less than r+1 bits
• widely used in practice (Ethernet, 802.11 WiFi, ATM)

• CRC example
• want: D.2r XOR R = nG
• equivalently: D.2r = nG XOR R
• equivalently: if we divide D.2r by G, want remainder R to satisfy:
• • $R = remainder[\frac{D.2^r}{G}]$

multiple access protocols

two types of “links”:

• point-to-point
• • PPP for dial-up access
• • point-to-point link between Ethernet switch, host
• broadcast (shared wire or medium)
• • old-fashioned Ethernet
• • upstream HFC
• • 802.11 wireless LAN

Multiple access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes: interference
• • collision if node receives two or more signals at the same time

multiple access protocol

• distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
• communication about channel sharing must use channel itself!
• • no out-of-band channel for coordination

An ideal multiple access protocol

• given: broadcast channel of rate R bps
• desiderata:
• • 1. when one node wants to transmit, it can send at rate R.
• • 1. when M nodes want to transmit, each can send at average rate R/M
• • 1. fully decentralized:
• • • no special node to coordinate transmissions
• • • no synchronization of clocks, slots
• • 1. simple

MAC protocols

taxonomy(分类)

three broad classes:

• channel partitioning
• • divide channel into smaller “pieces” (time slots, frequency, code)
• • allocate piece to node for exclusive use
• random access
• • channel not divided, allow collisions
• • “recover” from collisions
• “taking turns”
• • nodes take turns, but nodes with more to send can take longer turns

Channel partitioning MAC protocols

TDMA: time division multiple access

• access to channel in “rounds”
• each station gets fixed length slot (length = packet transmission time) in each round
• unused slots go idle
• example: 6-station LAN, 1,3,4 have packets to send, slots 2,5,6 idle

FDMA: frequency division multiple access

• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go idle
• example: 6-station LAN, 1,3,4 have packet to send, frequency bands 2,5,6 idle

Random access protocols

• when node has packet to send
• • transmit at full channel data rate R.
• • no a priori coordination among nodes
• two or more transmitting nodes ➜ “collision”,
• random access MAC protocol specifies:
• • how to detect collisions
• • how to recover from collisions (e.g., via delayed retransmissions)
• examples of random access MAC protocols:
• • slotted ALOHA
• • ALOHA
• • CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA

assumptions:

• all frames same size
• time divided into equal size slots (time to transmit 1 frame)
• nodes start to transmit only slot beginning
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes detect collision

operation:

• when node obtains fresh frame, transmits in next slot
• • if no collision: node can send new frame in next slot
• • if collision: node retransmits frame in each subsequent slot with prob. p until success

• Pros:
• single active node can continuously transmit at full rate of channel
• highly decentralized: only slots in nodes need to be in sync

• simple

• Cons:

• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less than time to transmit packet
• clock synchronization

efficiency: long-run fraction of successful slots (many nodes, all with many frames to send)

• suppose: N nodes with many frames to send, each transmits in slot with probability p
• prob that given node has success in a slot = p(1-p)N-1

• prob that any node has a success = Np(1-p)N-1

• max efficiency: find p* that maximizes Np(1-p)N-1

• for many nodes, take limit of Np(1-p)^{N-1} as N goes to infinity, gives:
• • max efficiency = 1/e = .37
• • at best: channel used for useful transmissions 37% of time

Pure (unslotted) ALOHA

• unslotted Aloha: simpler, no synchronization
• when frame first arrives
• • transmit immediately
• collision probability increases:
• • frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Pure ALOHA efficiency

P(success by given node) = P(node transmits) .
                                              P(no other node transmits in [t0-1,t0] .
                                              P(no other node transmits in [t0,t0+1]
                         = p . (1-p)^{N-1} . (1-p)^{N-1}  
                         = p . (1-p)^{2(N-1)}
                    … choosing optimum p and then letting n -> \infinite
                         = 1/(2e) = .18

• even worse than slotted Aloha

CSMA collisions

• collisions can still occur: propagation delay means two nodes may not hear each other’s transmission
• collision: entire packet transmission time wasted
• distance & propagation delay play role in determining collision probability

CSMA/CD (collision detection)

• CSMA/CD: carrier sensing, deferral as in CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing channel wastage

Ethernet CSMA/CD algorithm:

1. NIC receives datagram from network layer, creates frame

2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits.

3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame !

4. If NIC detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, NIC enters binary (exponential) backoff:

• after mth collision, NIC chooses K at random from {0,1,2, …, 2^m-1}. NIC waits K·512 bit times, returns to Step 2
• longer backoff interval with more collisions

CSMA/CD efficiency:

• $t_{prop}$ = max prop delay between 2 nodes in LAN
• $t_{trans}$ = time to transmit max-size frame
• $efficiency = \frac{1}{1 + 5t{prop}/t{trans}}$
• efficiency goes to 1
• • as $t_{prop}$ goes to 0
• • as $t_{trans}$ goes to infinity
• better performance than ALOHA: and simple, cheap, decentralized

“Taking turns” MAC protocols

• channel partitioning MAC protocols:
• • share channel efficiently and fairly at high load
• • inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!
• random access MAC protocols
• • efficient at low load: single node can fully utilize channel
• • high load: collision overhead
• “taking turns” protocols
• • look for best of both worlds!

• polling(轮询):

• • master node “invites” slave nodes to transmit in turn
• • typically used with “dumb” slave devices
• concerns:
• • polling overhead
• • latency
• • single point of failure (master)

• token(令牌) passing:
• • control token passed from one node to next sequentially.
• • token message
• • concerns:
• • • token overhead
• • • latency
• • • single point of failure (token)

Cable access network

• multiple 40Mbps downstream (broadcast) channels
• • single CMTS transmits into channels
• multiple 30 Mbps upstream channels
• • multiple access: all users contend for certain upstream channel time slots (others assigned)

• DOCSIS: data over cable service interface spec
• • FDM over upstream, downstream frequency channels
• • TDM upstream: some slots assigned, some have contention
• • • downstream MAP frame: assigns upstream slots
• • • request for upstream slots (and data) transmitted random access (binary backoff) in selected slots

Summary of MAC protocols

• channel partitioning, by time, frequency or code
• • Time Division, Frequency Division
• random access(dynamic),
• • ALOHA, S-ALOHA, CSMA, CSMA/CD
• • carrier sensing: easy in some technologies (wire), hard in others (wireless)
• • CSMA/CD used in Ethernet
• • CSMA/CA used in 802.11
• taking turns
• • polling from central site, token passing
• • Bluetooth, FDDI, token ring

LANs

MAC addresses

32-bit IP address:

• network-layer address for interface
• used for layer 3 (network layer) forwarding

MAC (or LAN or physical or Ethernet) address:

• function: used ‘locally” to get frame from one interface to another physically-connected interface (same network, in IP-addressing sense)
• 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable
• • e.g.: 1A-2F-BB-76-09-AD
• • hexadecimal (base 16) notation(each “numeral” represents 4 bits)

• each adapter on LAN has unique LAN address

• MAC address allocation administered by IEEE

• manufacturer buys portion of MAC address space (to assure uniqueness)

• analogy:

• • MAC address: like Social Security Number
• • IP address: like postal address
• MAC flat address ➜ portability
• • can move LAN card from one LAN to another
• IP hierarchical address not portable
• • address depends on IP subnet to which node is attached

ARP: address resolution protocol

Question: how to determine interface’s MAC address, knowing its IP address?

• ARP table: each IP node (host, router) on LAN has table
• • IP/MAC address mappings for some LAN nodes:
• • • < IP address; MAC address; TTL>
• • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

ARP protocol: same LAN

• A wants to send datagram to B
• • B’s MAC address not in A’s ARP table.
• A broadcasts ARP query packet, containing B’s IP address
• • destination MAC address = FF-FF-FF-FF-FF-FF
• • all nodes on LAN receive ARP query
• B receives ARP packet, replies to A with its (B’s) MAC address
• • frame sent to A’s MAC address (unicast)
• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
• • soft state: information that times out (goes away) unless refreshed
• ARP is “plug-and-play”(即插即用):
• • nodes create their ARP tables without intervention from net administrator

Addressing: routing to another LAN

walkthrough: send datagram from A to B via R

• focus on addressing – at IP (datagram) and MAC layer (frame)
• assume A knows B’s IP address
• assume A knows IP address of first hop router, R (how?)
• assume A knows R’s MAC address (how?)

• A creates IP datagram with IP source A, destination B
• A creates link-layer frame with R’s MAC address as destination address, frame contains A-to-B IP datagram

• frame sent from A to R
• frame received at R, datagram removed, passed up to IP

• R forwards datagram with IP source A, destination B
• R creates link-layer frame with B’s MAC address as destination address, frame contains A-to-B IP datagram

Ethernet

• “dominant” wired LAN technology:
• single chip, multiple speeds (e.g., Broadcom BCM5761)
• first widely used LAN technology
• simpler, cheap
• kept up with speed race: 10 Mbps – 10 Gbps

Ethernet: physical topology

• bus: popular through mid 90s
• • all nodes in same collision domain (can collide with each other)
• star: prevails today
• • active switch in center
• • each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)

Ethernet frame structure

• sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

• preamble:
• • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011
• • used to synchronize receiver, sender clock rates

• addresses: 6 byte source, destination MAC addresses

• • if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol
• • otherwise, adapter discards frame
• type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
• CRC: cyclic redundancy check at receiver
• • error detected: frame is dropped

Ethernet: unreliable, connectionless

• connectionless: no handshaking between sending and receiving NICs
• unreliable: receiving NIC doesn’t send acks or nacks to sending NIC
• • data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost
• Ethernet’s MAC protocol: unslotted CSMA/CD with binary backoff

802.3 Ethernet standards: link & physical layers

• many different Ethernet standards
• • common MAC protocol and frame format
• • different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10 Gbps, 40 Gbps
• • different physical layer media: fiber, cable

Ethernet switch

• link-layer device: takes an active role
• • store, forward Ethernet frames
• • examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment
• transparent
• • hosts are unaware of presence of switches
• plug-and-play, self-learning
• • switches do not need to be configured

Switch: multiple simultaneous transmissions

• hosts have dedicated, direct connection to switch
• switches buffer packets
• Ethernet protocol used on each incoming link, but no collisions; full duplex
• • each link is its own collision domain
• switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions

Switch forwarding table

Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?

• A: each switch has a switch table, each entry:
• • (MAC address of host, interface to reach host, time stamp)
• • looks like a routing table

Switch: self-learning

Q: how are entries created, maintained in switch table? something like a routing protocol?

• switch learns which hosts can be reached through which interfaces
• • when frame received, switch “learns” location of sender: incoming LAN segment
• • records sender/location pair in switch table

Switch: frame filtering/forwarding

when frame received at switch:

1. record incoming link, MAC address of sending host

2. index switch table using MAC destination address

3. if entry found for destination then {
       if destination on segment from which frame arrived then 
           drop frame
       else 
           forward frame on interface indicated by entry
       }   
   else 
       flood  /* forward on all interfaces except arriving interface */

example:

• frame destination, A’, location unknown: flood
• destination A location known: selectively send on just one link

Interconnecting switches

self-learning switches can be connected together:

Q: sending from A to G - how does S1 know to forward frame destined to G via S4 and S3?

• A: self learning! (works exactly the same as in single-switch case!)

Self-learning multi-switch example:

• Suppose C sends frame to I, I responds to C

Q: show switch tables and packet forwarding in S1, S2, S3, S4

• ?

Switches vs. routers

• both are store-and-forward:
• • routers: network-layer devices (examine network-layer headers)
• • switches: link-layer devices (examine link-layer headers)

• both have forwarding tables:

• • routers: compute tables using routing algorithms, IP addresses
• • switches: learn forwarding table using flooding, learning, MAC addresses

VLANs: motivation

• consider:
• CS user moves office to EE, but wants connect to CS switch?
• single broadcast domain:
• • all layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN
• • security/privacy, efficiency issues

Virtual Local Area Network: switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.

port-based VLAN: switch ports grouped (by switch management software) so that single physical switch operates as multiple virtual switches

• traffic isolation: frames to/from ports 1-8 can only reach ports 1-8
• • can also define VLAN based on MAC addresses of endpoints, rather than switch port

• dynamic membership: ports can be dynamically assigned among VLANs

• forwarding between VLANS: done via routing (just as with separate switches)

• • in practice vendors sell combined switches plus routers

VLANS spanning multiple switches

• trunk port: carries frames between VLANS defined over multiple physical switches
• • frames forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info)
• • 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports

802.1Q VLAN frame format

link virtualization: MPLS

Multiprotocol label switching (MPLS):

• initial goal: high-speed IP forwarding using fixed length label (instead of IP address)
• • fast lookup using fixed length identifier (rather than shortest prefix matching)
• • borrowing ideas from Virtual Circuit (VC) approach
• • but IP datagram still keeps IP address!

MPLS capable routers

• a.k.a. label-switched router
• forward packets to outgoing interface based only on label value (don’t inspect IP address)
• • MPLS forwarding table distinct from IP forwarding tables

MPLS versus IP paths

• IP routing: path to destination determined by destination address alone
• MPLS routing: path to destination can be based on source and destination address
• • fast reroute: precompute backup routes in case of link failure

MPLS signaling:

• modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing, e.g., link bandwidth, amount of “reserved” link bandwidth

• entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

MPLS forwarding tables: