Introduction 1 Lecture 26 Link Layer (PPP, Virtualization) slides are modified from J. Kurose & K. Ross University of Nevada – Reno Computer Science & Engineering Department Fall 2011 CPE 400 / 600 Computer Communication Networks

5: DataLink Layer5-2 VLANs: motivation What happens if: r CS user moves office to CE, but wants connect to CS switch? r single broadcast domain: m all layer-2 broadcast traffic (ARP, DHCP) crosses entire LAN security/privacy, efficiency issues r each lowest level switch has only few ports in use Computer Science Electrical Engineering Computer Engineering What’s wrong with this picture?

5: DataLink Layer5-3 VLANs Port-based VLAN: switch ports grouped (by switch management software) so that single physical switch …… Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. Virtual Local Area Network … Computer Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … Computer Engineering (VLAN ports 1-8) … … Computer Science (VLAN ports 9-16) … operates as multiple virtual switches

5: DataLink Layer5-4 Port-based VLAN … Computer Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … r traffic isolation: frames to/from ports 1-8 can only reach ports 1-8 m can also define VLAN based on MAC addresses of endpoints, rather than switch port r dynamic membership: ports can be dynamically assigned among VLANs router r forwarding between VLANS: done via routing r just as with separate switches m in practice vendors sell combined switches plus routers

5: DataLink Layer5-5 VLANS spanning multiple switches r trunk port: carries frames between VLANS defined over multiple physical switches m frames forwarded within VLAN between switches can’t be vanilla frames (must carry VLAN ID info) m 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports … Computer Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) 15 … Ports 2,3,5 belong to CE VLAN Ports 4,6,7,8 belong to CS VLAN

5: DataLink Layer5-6 Type 2-byte Tag Protocol Identifier (value: 81-00) Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) Recomputed CRC 802.1Q VLAN frame format frame 802.1Q frame

5: DataLink Layer5-7 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link virtualization: MPLS

5: DataLink Layer5-8 Point to Point Data Link Control r one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line r popular point-to-point DLC protocols: m PPP: point-to-point protocol m HDLC: High level data link control Data link used to be considered “high layer” in protocol stack!

5: DataLink Layer5-9 PPP Design Requirements [RFC 1557] r packet framing: encapsulation of network-layer datagram in data link frame m carry network layer data of any network layer protocol (not just IP) at same time m ability to demultiplex upwards r bit transparency: must carry any bit pattern in the data field r error detection (no correction) r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address

5: DataLink Layer5-10 PPP non-requirements r no error correction/recovery r no flow control r out of order delivery OK r no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

5: DataLink Layer5-11 PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; m in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered m eg, PPP-LCP, IP, IPCP, etc

5: DataLink Layer5-12 PPP Data Frame r info: upper layer data being carried r check: cyclic redundancy check for error detection

5: DataLink Layer5-13 Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag? r Sender: adds (“stuffs”) extra byte before each data byte r Receiver: m discard first byte, continue data reception m single : flag byte

5: DataLink Layer5-14 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

5: DataLink Layer5-15 PPP Data Control Protocol Before exchanging network-layer data, data link peers must r configure PPP link m max. frame length, authentication r learn/configure network layer information m for IP: carry IP Control Protocol (IPCP) msgs to configure/learn IP address

5: DataLink Layer5-16 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link virtualization: MPLS

5: DataLink Layer5-17 Virtualization of networks Virtualization of resources: powerful abstraction in systems engineering: r computing examples: virtual memory, virtual devices m Virtual machines: e.g., java m IBM VM os from 1960’s/70’s r layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

5: DataLink Layer5-18 The Internet: virtualizing networks 1974: multiple unconnected nets m ARPAnet m data-over-cable networks m packet satellite network (Aloha) m packet radio network … differing in: m addressing conventions m packet formats m error recovery m routing ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp

5: DataLink Layer5-19 The Internet: virtualizing networks ARPAnet satellite net gateway Internetwork layer (IP): r addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity r network of networks Gateway: r “embed internetwork packets in local packet format or extract them” r route (at internetwork level) to next gateway

5: DataLink Layer5-20 Cerf & Kahn’s Internetwork Architecture What is virtualized? r two layers of addressing: internetwork and local network r new layer (IP) makes everything homogeneous at internetwork layer r underlying local network technology m cable m satellite m 56K telephone modem m today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP!

5: DataLink Layer5-21 ATM and MPLS r ATM, MPLS separate networks in their own right m different service models, addressing, routing from Internet r viewed by Internet as logical link connecting IP routers m just like dialup link is really part of separate network (telephone network) r ATM, MPLS: of technical interest in their own right

5: DataLink Layer5-22 Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed m 155Mbps to 622 Mbps and higher m Broadband Integrated Service Digital Network architecture r Goal: integrated, end-end transport of carry voice, video, data m meeting timing/QoS requirements of voice, video versus Internet best-effort model m “next generation” telephony: technical roots in telephone world m packet-switching using virtual circuits fixed length packets, called “cells”

5: DataLink Layer5-23 Multiprotocol label switching (MPLS) r initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding m borrowing ideas from Virtual Circuit (VC) approach m but IP datagram still keeps IP address! PPP or Ethernet header IP header remainder of link-layer frame MPLS header label Exp S TTL

5: DataLink Layer5-24 MPLS capable routers r a.k.a. label-switched router r forwards packets to outgoing interface based only on label value (don’t inspect IP address) m MPLS forwarding table distinct from IP tables r signaling protocol needed to set up forwarding m RSVP-TE m forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! m use MPLS for traffic engineering r must co-exist with IP-only routers

5: DataLink Layer5-25 R1 R2 D R3 R4 R A R6 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A D 0 in out out label label dest interface 10 A 0 12 D 0 1 in out out label label dest interface 8 6 A A 1 MPLS forwarding tables

5: DataLink Layer5-26 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing r instantiation and implementation of various link layer technologies m Ethernet m switched LANS, VLANs m PPP m virtualized networks as a link layer: MPLS