Now Arriving at Layer 3. Packet Forwarding. Router Design. Network Layers and Routers. Switching and Forwarding. Forwarding

Transcription

1 Now rriving at Layer Packet orwarding although layer switches and layer routers are similar in many ways and TM/Virtual are used at layer these days 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 Layers and Routers Router esign pplication Presentation pplication Presentation Input Input Output Output Session Transport Router Session Transport Input Input abric Output Output ata Link ata Link ata Link Input Output Input Output 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 4 orwarding ing and orwarding orwarding lgorithm onsult packet header onsult forwarding tables ecide on output port Three general types atagram forwarding Virtual Source Routing iffer by contents of header and tables orwarding The task of specifying an appropriate output port for a packet atagram Virtual ircuit ing Source Routing ach packet contains enough information for a switch to determine the correct output port Later uilding forwarding tables routing. Packet eader Output Specification 9/7/6 S/ 48 - UIU, all 6 5 9/7/6 S/ 48 - UIU, all 6 6

2 orwarding with atagrams orwarding with atagrams onnectionless ach packet travels independently Translates global address to output port Maintains table of translations Used in traditional data networks i.e., Internet α β γ δ 9/7/6 S/ 48 - UIU, all 6 7 9/7/6 S/ 48 - UIU, all 6 8 Routing Table orwarding with atagrams ach switch maintains a routing table that translates a host name to an output port α β δ γ α s Table β s Table γ s Table δ s Table α β δ γ What happens to the last packet? sends: sends: sends: sends: sends: T T T T T 9/7/6 S/ 48 - UIU, all 6 9 9/7/6 S/ 48 - UIU, all 6 orwarding with atagrams Traceroute xample nalogous to following signs Requires globally unique addresses Routing is decentralized router follows global routing algorithms Two packets usually take the same path but ach router can change its mind at any time rom W solutions traceroute traceroute to ( ), hops max, 4 byte packets uiuc-ewsl-vlan.gw.uiuc.edu (.6.6.).45 ms. ms.9 ms ae--.bbr.washington.level.net ( ).946 ms as--.bbr.washington.level.net (9.47..).5 ms.8 ms 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6

3 atagrams dvantages Routes around failures an send traffic immediately isadvantages eader requires full unique address Might not be possible to deliver packet Successive packets may not follow the same route lobal address to path translations requires significant storage onnection oriented Requires explicit setup and teardown Packets follow established route Why support connections in a network? Useful for service notions Important for telephony Translates virtual circuit I on incoming link to virtual circuit I on outgoing link ircuit Ids can be per-link or per-switch Used in TM 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 4 Virtual Packet header stores: Virtual ircuit I Router stores: Table of how to forward packets for each virtual circuit Note: need not be global ssign a to a circuit for each linklink pair Set up virtual circuit identifier () is assigned to the circuit for each link it traverses is locally significant <incoming port, incoming > uniquely identifies V Maintains a translation table from <incoming port, incoming > to <outgoing port, outgoing > Permanent Virtual (PV) Long-lived Virtual (SV) Uses signaling to establish V 9/7/6 S/ 48 - UIU, all 6 5 9/7/6 S/ 48 - UIU, all 6 6 simple example setup protocol ach host and switch maintains per-link local variable for assignment When setup frame leaves host/switch ssign outgoing Increment assignment counter port and circuit id combination is unique switches maintain translation table from incoming port/ pair to outgoing port/ pair ssumptions are simplex On a duplex link, the same can be used for two circuits, one in each direction The same can be used on different ports of the same switch t setup, the lowest available is used 9/7/6 S/ 48 - UIU, all 6 7 9/7/6 S/ 48 - UIU, all 6 8

4 Set up circuit: Setup est =? α β γ Setup <,> <,?> Setup <,> est = <,?> est = δ Setup <,> <,?> est = 9/7/6 S/ 48 - UIU, all 6 9 Set up circuit: K = α β γ K <,> <,> K <,> = <,> = δ K <,> <,> = = 9/7/6 S/ 48 - UIU, all 6 α β γ δ Set up circuits: α β δ α γ δ β Table entries after connection is set 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 Table entries after,, connection is set α δ β γ nalogous to a game of following a sequence of clues dvantages eader (for a data packet) requires only virtual circuit I onnection request contains global address an reserve resources at setup time isadvantages Typically must wait one RTT for setup annot dynamically avoid failures, must reestablish connection lobal address path information still necessary for connection setup 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 4 4

5 orwarding with source routing orwarding with Source Routing Packet header specifies directions One direction per switch bsolute name Next switch name Relative Turn clockwise ports es may delete or rotate directions within packet headers No state stored at switch! α β δ What happens to the last packet? γ sends: T sends: T sends: T sends: T sends: T 9/7/6 S/ 48 - UIU, all 6 5 9/7/6 S/ 48 - UIU, all 6 6 orwarding with Source Routing TM nalogous to following directions dvantages Simple switches ast and cheap isadvantages s must know entire topology hanges must propagate to all hosts eaders might get large efined by the TM orum ormed October 99 Joint effort of the telephony and data network industry igh-level Overview Virtual circuit routing ixed length frames (aka cells) Standard define layers 9/7/6 S/ 48 - UIU, all 6 7 9/7/6 S/ 48 - UIU, all 6 8 TM TM etails TL daptation Layer (L) onvergence sub-layer (S) supports different application service models Segmentation and reassembly (SR) Supports variable-length frames TM Layer Virtual circuits maintenance ell header generation low control Layer Transmission convergence (T) rror detection, raming medium dependent (PM) sublayer encoding S SR T PM L TM phys. Where is TM used? ommon in WNs an also be used in LNs What is TM built on? Typically implemented on SONT esign onnection establishment Signaling (Q.9) Virtual circuits Virtual Paths undles of virtual circuits Share common route Optimizes forwarding 9/7/6 S/ 48 - UIU, all 6 9 9/7/6 S/ 48 - UIU, all 6 5

6 TM ells TM Rationale ell specification 5-bytes 48-byte payload -byte R 4-byte header Why hierarchical connections? Setup New virtual circuits can follow existing virtual path routes orwarding Virtual Path Identifier (VPI) Used between switches Virtual ircuit Identifier () Used for last hop Routing around failures Need only change virtual path once for 64K virtual circuits 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 TM Rationale TM Rationale Public Why fixed-length frames? ardware Simpler processing for known frame sizes Parallelization of processing stages Is there an optimal length? Small cells igh header-to-data overhead Large frames must be fragmented Large cells Low utilization for small messages 9/7/6 S/ 48 - UIU, all 6 9/7/6 S/ 48 - UIU, all 6 4 TM Rationale TM Rationale Why short cells? etter queueing behavior Reduced granularity of preemption igh priority cell waits for one cell Long cell: potentially long wait Short cell: limited wait Limits end-to-end jitter (variance in latency) Shorted store-and-forward delay es typically store whole frame, then forward Short cells enable first part of a fragmented frame to be sent while the rest is still arriving Queueing ehavior xample onsider 4K vs. 5 cells, Mbps Link Preemption igh priority cell arrives just as switch starts sending low-priority cell 4K: high-priority cell must wait for 8µs 5: high priority cell must wait for 4µs Queueing Two 4K frames arrive simultaneously at time 4K: link is idle until all data arrives at time 8µs, 8K left to send 5: irst 5 is sent at time 4µs. t time 8µs, 4K left to send 9/7/6 S/ 48 - UIU, all 6 5 9/7/6 S/ 48 - UIU, all 6 6 6

7 TM Rationale TM and LNs Why 5-byte cells? US wanted 64-bytes igital encoding for voice frame = 64Kbps (8-bit samples, 8Khz) ollect one sample per frame With 64-byte cells, no need for echo cancellation Latency cell = 6msec Not detectable by humans urope wanted -byte Shorter distances, no need for echo cancellation ompromise 48-bytes of data! Problems with 5-byte (48-bytes of data) cells Not a power of! omments ed networks have better performance then shared media Shared media performance is increasing (-Mps and igabit thernet) TM in a LN TM doesn t look like a traditional LN Specifically, no native support for broadcast and multicast Solution Redesign protocols that require broadcast/multicast Make TM behave more like a shared medium LN mulation (LN) 9/7/6 S/ 48 - UIU, all 6 7 9/7/6 S/ 48 - UIU, all 6 8 Structure of LN TM Local rea mulation (LN) TM network can have multiple mulated LN s (LN s) ach LN corresponds to a single network s do not have to be geographically oriented s can move between buildings, but remain on the same network ccess ontrol Lists (L s) on LN servers ontrol which hosts can join which LN s TM LN TM LN LN- TM TM LN- LN/thernet daptor ard LN/thernet daptor ard ll same color hosts think they are on the same thernet thernet thernet 9/7/6 S/ 48 - UIU, all 6 9 9/7/6 S/ 48 - UIU, all 6 4 TM Local rea mulation (LN) TM/LN Protocol Layers TM LN/thernet daptor ard thernet igher Layer Protocols (IP, RP ) igher Layer Protocols (IP, RP ) TM LN/thernet daptor ard ll hosts think they are on the same thernet thernet Signaling and LN L 5 TM thernet-like Interface PY TM PY Signaling and LN L 5 TM 9/7/6 S/ 48 - UIU, all 6 4 9/7/6 S/ 48 - UIU, all 6 4 7