The Data Link Layer Our goals: Previous understand principles - PDF document

The Data Link Layer Our goals: Previous ❍ understand principles ❍ link layer services behind data link layer ❍ error detection, correction services: ❍ multiple access protocols and • error detection, LANs correction ❍ link layer addressing, ARP • sharing a broadcast ❍ Ethernet channel: multiple access ❍ Token Ring • Link layer addressing ❍ hubs, bridges, switches • reliable data transfer, ❍ PPP flow control: done! Today ❒ instantiation and implementation of various ❒ ATM link layer technologies ❒ X.25 and Frame Relay 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 1 Asynchronous Transfer Mode: ATM ❒ 1980s/1990s: standard for high-speed Broadband Integrated Service Digital Network architecture ❒ Goal: integrated, end-end transport of voice, video, data ❍ meeting timing/QoS requirements of voice, video ❍ “next generation” telephony ❍ packet-switching using virtual circuits 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 2 ATM architecture ❒ adaptation layer: only at edge of ATM network ❍ data segmentation/reassembly ❍ roughly analogous to Internet transport layer ❒ ATM layer: “network” layer ❍ cell switching, routing ❒ physical layer 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 3 1

ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” ❍ ATM is a network technology Reality: used to connect IP backbone routers ❍ “IP over ATM” ❍ ATM as switched link layer, connecting IP routers 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 4 ATM Physical Layer Two pieces (sublayers) of physical layer: ❒ Transmission Convergence Sublayer (TCS) ❍ adapts ATM layer above to PMD sublayer below ❍ Header checksum generation: 8 bits CRC ❍ Cell delineation ❍ With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send ❒ Physical Medium Dependent ❍ depends on physical medium being used ❍ with or without transmission frame structure ❍ bit generation and delineation 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 5 ATM Layer Service: transport cells across ATM network ❒ analogous to IP network layer ❒ very different services than IP network layer Guarantees ? Network Service Congestion Architecture Model Bandwidth Loss Order Timing feedback Internet best effort none no no no no (inferred via loss) ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion guaranteed no ATM ABR yes no yes minimum ATM UBR none no yes no no 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 6 2

ATM Layer: Virtual Circuits ❒ VC transport: cells carried on VC from source to dest ❍ a VC must exist before data can flow ❍ each packet carries VC identifier (not destination ID) ❍ every switch on source-dest path maintain “state” for each passing connection ❍ link, switch resources (bandwidth, buffers) may be allocated to VC ❒ Permanent VCs (PVCs) ❍ long lasting connections ❍ typically: “permanent” route between IP routers ❒ Switched VCs (SVC): ❍ dynamically set up on per-call basis 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 7 ATM VCs ❒ Advantages of ATM VC approach: ❍ QoS performance guarantees ❒ Drawbacks of ATM VC approach: ❍ one PVC between each source/dest pair) does not scale (N*2 connections needed) ❍ SVC introduces call setup latency, processing overhead for short lived connections 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 8 ATM Layer: ATM cell ❒ 5-byte ATM cell header ❒ 48-byte payload Cell header Cell format 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 9 3

ATM Adaptation Layer (AAL) ❒ ATM Adaptation Layer (AAL): “adapts” upper layers to ATM layer below ❒ AAL present only in end systems ❒ AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells ❍ analogy: TCP segment in many IP packets 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 10 ATM Adaption Layer [more] Different versions of AAL layers, depending on ATM service class: ❒ AAL1: for CBR services, e.g. circuit emulation ❒ AAL2: for VBR services, e.g., MPEG video ❒ AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 11 AAL5 - Simple And Efficient ❒ AAL5 : low overhead AAL used to carry IP datagrams ❍ 4 byte cyclic redundancy check ❍ PAD ensures payload multiple of 48 bytes ❍ large AAL5 data unit to be fragmented into 48- byte ATM cells ❍ no CPCS header, no AAL header or trailer • low OH 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 12 4

IP-Over-ATM Classic IP only IP over ATM ❒ 3 “networks” (e.g., ❒ replace “network” LAN segments) with ATM network ❒ MAC (802.3) and IP ❒ ATM addresses, IP addresses addresses ATM network Ethernet Ethernet LANs LANs 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 13 Datagram Journey in IP-over- ATM Network ATM A network Ethernet B LANs 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 14 ARP in ATM Nets ❒ ATM network needs destination ATM address ❒ IP/ATM address translation done by ATM ARP (Address Resolution Protocol) ❍ hosts register their own ATM addresses with the ARP server ❍ hosts asks the ARP server for other hosts ATM address 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 15 5

ATM ABR congestion control RM (resource management) ABR: available bit rate: cells: ❒ “elastic service” ❒ sent by sender, interspersed ❒ if sender’s path with data cells “underloaded”: ❒ bits in RM cell set by switches ❍ sender should use (“ network-assisted” ) available bandwidth ❍ NI bit: no increase in rate ❒ if sender’s path (mild congestion) congested: ❍ CI bit: congestion ❍ sender throttled to indication minimum guaranteed ❒ RM cells returned to sender by receiver, with bits intact rate 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 16 ATM ABR congestion control ❒ two-byte ER (explicit rate) field in RM cell ❍ congested switch may lower ER value in cell ❒ EFCI bit in data cells: set to 1 in congested switch ❍ if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 17 X.25 and Frame Relay Like ATM: ❒ wide area network technologies ❒ virtual circuit oriented ❒ origins in telephony world ❒ can be used to carry IP datagrams ❍ can thus be viewed as Link Layers by IP protocol 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 18 6

X.25 ❒ X.25 builds VC between source and destination for each user connection ❒ Per-hop control along path ❍ error control (with retransmissions) on each hop using LAP-B • variant of the HDLC protocol ❍ per-hop flow control using credits • congestion arising at intermediate node propagates to previous node on path • back to source via back pressure 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 19 IP versus X.25 ❒ X.25: reliable in-sequence end-end delivery from end-to-end ❍ “intelligence in the network” ❒ IP: unreliable, out-of-sequence end- end delivery ❍ “intelligence in the endpoints” ❒ gigabit routers: limited processing possible ❒ 2000: IP wins 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 20 Frame Relay ❒ Designed in late ‘80s, widely deployed in the ‘90s ❒ Frame relay service: ❍ no error control ❍ end-to-end congestion control 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 21 7

Frame Relay (more) ❒ Designed to interconnect corporate customer LANs ❍ typically permanent VC’s: “ pipe ” carrying aggregate traffic between two routers ❍ switched VC’s: as in ATM ❒ corporate customer leases FR service from public Frame Relay network 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 22 Link Layer: Summary ❒ principles behind data link layer services: Application ❍ error detection, correction ❍ sharing a broadcast channel: multiple access UDP TCP ❍ link layer addressing, ARP ❒ various link layer technologies IP ❍ Ethernet ❍ hubs, bridges, switches Link ❍ PPP ❍ ATM Physical ❍ X.25, Frame Relay ❒ journey down the protocol stack now OVER! ❍ Next stops: multimedia, security, network management 12 October 2001 Data Communications, Jonny Pettersson, UmU Link 3 23 8