Interconnection Protocols Interconnection Protocols Purpose of Course Good Ideas • Understand generic problems and approaches • Parameters (e.g., how often to send hellos) • Less important (for this course) but covered should be settable at nodes one at a time somewhat: description of what happens to be without having to reset the whole network implemented • How can you ensure that two nodes have compatible parameters? • Some changes are “compatible” in that they can be added to nodes one at a time without disrupting operations. • How can you design messages so that you can add fields in the future and still interoperate with old nodes? 1 2 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Another Distance Vector Link Costs Trick • Some think it’s a good idea for the cost of the • Double metric: hops and cost link to vary according to congestion. - - allows routes to be chosen based on “real” route around congestion metric (allowing slow links to have higher - don’t have to configure link costs costs) • I don’t - detects unreachable destinations (with path - extra routing control traffic lengths more than 15) just as quickly - less time the network is in a converged state - can’t react quickly enough to matter 3 4 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols IEEE 802 What is a LAN? Chartered to standardize LANs Badly defined term (typical in this field) Took their job very seriously -- standardized lots Assumed general characteristics: of LANs • multiaccess • logical full connectivity • 802.1 -- management, interconnection • multicast/broadcast capability • 802.2 -- SAPs, LLC • limited scalability (distance, number of • 802.3 -- CSMA/CD stations, total traffic) X • 802.4 -- Token bus A • 802.5 -- Token ring Q • Other committees: FDDI, Metropolitan area nets, security M 5 6 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols What is Multicast? Taking Turns • Unicast: Alice sends to Bob • Suppose there’s a shared medium (like the • But, on a LAN, everyone can hear, so if they’re room we’re in). If more than one speaker, just not Bob, they discard the packet hear garbage • • The hardware has to be smart to filter out Old kinds had a master that polled the “slaves” traffic not intended for this node (so as not to - “Alice, do you have anything to send?” bother the node with all the interrupts) - Alice “Terminal 23 typed ‘Gree’” • But sometimes you want to send to a group of - “Amy, I’m sending 315 to you. Anything to nodes. Use the natural ability of the LAN! send?” - to send to a group of nodes...tell them all to - listen to address X (in addition to their own address). Then transmit to X and they’ll all - “Andy do you have anything to send?” receive it - Fancier protocols poll recently active • “Promiscuous” listen means you tell your chip stations more frequently to send everything to you. - Could have 2 slaves talking directly, but only if 2 addresses in header 7 8 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols CSMA/CD (Ethernet) The “CD” in CSMA/CD - Carrier Sense Multicast Access with • Speed of light: “It’s not just a good idea. It’s Collision Detect. the law” - Single “bus” on which all stations reside. If • Suppose packet very small. May not detect anyone transmits, everyone hears collision - Intended for peer-to-peer (not master to • Maximum length of bus (maximum delay, slaves) including repeaters), and minimum sized - To transmit: If link idle, transmit. Check for packet so guaranteed to detect delay. collision. If collision, back off a random amt. • Packet size must be twice length of cable in If again a collision, back off a random amt order for sender to know it hasn’t collided! chosen from a double-sized interval, etc., (why?) until max collisions. Then give up on that • Hub: Multi-port repeater packet, but next packet start from small interval again “exponential backoff” - Many variants 9 10 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Token Ring Token Rings, Continued C B D • Transmitter removes frame from ring (why?) A E • Slower rings (802.5) tend to have one frame at I a time H F • Faster rings (e.g., FDDI) have a train of G packets followed by the token • Idea simple, but made much more complicated • Just a bunch of point-to-point links with priorities • Want to minimize delay around the circle • Minimum delay: one bit per station • “token” can be turned into “start of packet” by flipping one bit • Token travels around ring. To transmit, when see token, flip bit, transmit packet • Two “ack” bits: A=address recognized, C=frame copied 11 12 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Token Bus Which is best? • Like Ethernet, everything on one wire • For awhile, hotly debated • Complicated protocol for building a logical • These days, all of them dying out! (I’ll show ordering of active stations you what “Ethernet” is these days in a later • Each active station must know predecessor, lecture) successor • When get token, send packet (if you have one), then transmit token to successor • Periodically, invite other stations to join the ring (between you and your successor) • VERY complex protocol for doing that 13 14 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols IEEE802 Addresses Address Issues • Idea: unique ID for each IEEE802 device • How do you write down an address? OUI - canonical written form: 43-75-cf-5f-45-7a • Transmitted on 802.3 or 802.4 LSB first group/individual 01000011 01110101 11001111 01011111 01000101 01111010 globally/locally assigned 24 • Transmitted on 802.5 MSB first • Assigned in blocks of 2 • Given 23 byte constant (Organizationally 11000010 10101110 11110011 11111010 10100010 01011110 Unique Identifier), plus group/individual bit • All 1’s intended to mean “broadcast”, i.e., • Group/individual bit supposed to be first bit “everyone”, which is nonsense. Really each transmitted protocol should use its own multicast address • Different bit order very annoying to mean all nodes that speak that protocol 15 16 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols EUI-48/EUI-64 Multi-Lingual Environments • EUI-48 is the 6-byte address already discussed • EUI-64 is IEEE’s new 8-byte address. • You can speak lots of things (IP, CLNP, IPX, • How much should be OUI? AppleTalk, etc.) • - 5 byte OUI, virtually unlimited number of Someone hands you a pile of bits. What is it? OUIs, 224 addresses per block - Maybe we were careful -- yeah, right - 3 byte OUI, virtually unlimited number of - Maybe we were lucky -- yeah, right addresses per OUI, but 22 bits (4 million) • Conclusion: not enough information in the OUIs packet header to differentiate -- need an extra - Consequence of too small block...some field in the data link header to say what it is people need to come back for more blocks - protocol type: well-known (globally - Consequence of too small OUI...run out of administered) values, one field in header OUIs, world ends - SAP (service access point) or socket: locally - They kept 3-byte OUI, but with plea to not administered, one for destination, one for ask for more OUIs until you’ve used up all source your addresses 17 18 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Packet HDRs on CSMA/CD How the SAPs work • Notice the “global/local” bit -- those SAPs are globally assigned! If you are a very privileged Ethernet protocol, and obtain one of these, you’d set DSAP=SSAP= your assigned SAP value 6 6 2 46-1500 4 • dest src p-t DATA fcs How does it work if you’re not a privileged protocol? Uh... 802.3 • World class kludge -- get a SAP value assigned 6 6 2 1 1 1 43-1497 4 to mean “underprivileged protocol”. That was dest src ln dsap ssap ctl data fcs done. It’s called SNAP SAP (SubNetwork Access Protocol), and it = aa hex. Format of SAP • If DSAP=SSAP=aa hex, then after CTL is a protocol type field G/L G/I • The protocol type is 5 bytes long • Convention: 0.0.0.protocol type allows 2 octet Ethertypes to fit into 5 octets 19 20 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Converting between 802.3 Why Bridges? and Ethernet • LANs don’t scale - Distance Ethernet - Number of stations 6 6 2 46-1500 - Traffic dest src X DATA • Want to glue LANs together 802.3 • Want local traffic to stay local 6 6 2 1 1 1 5 43-1497 dest src ln AA AA 3 0.0.0.X data AC D 21 22 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Two Types of Bridges Transparent Bridge • Design constraint: work with unmodified • Two types stations, designed to work on single LAN - Transparent (also known as spanning tree) - Source routing • Transparent standardized by 802.1 • Source routing originally adopted by 802.5 AC D when it lost out to transparent bridges in 802.1 • Packet contains two interesting fields: - destination - source • Learn based on source field • Forward based on destination field • When in doubt, forward 23 24 Copyright © 2001 Radia Perlman Copyright © 2001 Radia Perlman Interconnection Protocols Interconnection Protocols Multiple