CEN 5501C - Computer Networks - Spring UF/CISE - Newman1 Computer Networks Chapter 4 – Source Routing Bridges

CEN 5501C - Computer Networks - Spring UF/CISE - Newman2 Why Source Routing? It exists –802 standards –IP option –GWEN –MANETs Instructive alternative –Variety provides insight –Variety provides options

CEN 5501C - Computer Networks - Spring UF/CISE - Newman3 Pure Source Routing Packet header contains route How does the source get the route? –Manually set –Cached from history –Discovered How to indicate source routed packet? –No field set aside for this in LAN header –Use “useless” bit…which one? Data Dest AddrSrc Addr

CEN 5501C - Computer Networks - Spring UF/CISE - Newman4 Routing Information (RI) Magic bit –Multicast bit in source address field (never send from a multicast address!) DataDest AddrSrc Addr RI Dest AddrSrc Addr Data Multicast bit = 0 Multicast bit = 1 Normal packet (transparent bridge routing): Source routed packet with Routing Information field:

CEN 5501C - Computer Networks - Spring UF/CISE - Newman5 RI Fields Type (3 bits) –Specifically routed (route in header) –All paths explorer (collect route as traversed) –Spanning tree explorer (only traverse ST) Length (5 bits) = # bytes in RI field Direction (1 bit) = traverse route L to R? Largest Frame (3 bits) –Represents one of a few popular packet sizes (516, 1500, 2052, 4472, 8144, 11407, 17800, 65535) Route (variable, multiple of 2 octets)

CEN 5501C - Computer Networks - Spring UF/CISE - Newman6 Route Designators 2-octet sub-fields of Route field in RI –12-bit LAN number –4-bit bridge number LAN numbers –Unique over all LANs in extended LAN Bridge Numbers –Unique for all bridges that connect the same pair of LANs

CEN 5501C - Computer Networks - Spring UF/CISE - Newman7 Bridge Numbers Suppose route were only LAN numbers… B1 B2 B3 B4 B5 LAN A LAN B … How many copies show up on LAN B?

CEN 5501C - Computer Networks - Spring UF/CISE - Newman8 Bridge Numbers Suppose there were multiple layers… LAN ALAN C … now how many copies show up on LAN C? LAN B

CEN 5501C - Computer Networks - Spring UF/CISE - Newman9 Bridge Numbers Single bridge number per bridge Can duplicate, but not for two bridges connecting same pair of LANs Easier if bridge could have different number per pair of LANs it connects, but –Not standard –Does not scale well (N 2 bridge numbers for N port bridge) Can make due with “internal LAN” if needed –Pretend each port connects to hallucinated LAN –Internal LAN will always have unique number, so no collisions across bridges! Last bridge number in route always “0”

CEN 5501C - Computer Networks - Spring UF/CISE - Newman10 Why Parallel Bridges? Robustness Underpowered bridges Low capacity links –Need SR to use both –Best if both links between same bridges LAN ALAN B LAN A LAN B LAN ALAN B

CEN 5501C - Computer Networks - Spring UF/CISE - Newman11 SR Bridge Frames 4 Frame types to handle –Transparent frames (no RI field) –Specifically routed –All paths explorer –Spanning tree explorer To each his own –Transparent bridges ignore SR frames –Pure SR bridges ignore transparent frames

CEN 5501C - Computer Networks - Spring UF/CISE - Newman12 Specifically Routed Frames SR Bridge B gets SR Frame on port P1 from LAN #X (to B, anyway) B sees SR frame, checks direction bit B forwards to port P2 iff –X is on the route, followed by Bridge # N, followed by LAN #Y –B associates LAN #Y with P2 –B’s bridge # w.r.t. (P1,P2) is N –Y appears only once on the route

CEN 5501C - Computer Networks - Spring UF/CISE - Newman13 All Paths Explorer Frames Source host first sends with no route info SR Bridge B gets APEx Frame on port P1 from LAN #X (to B, anyway) If no route info yet, B does for every port P other than P1 –Initialize route to [X,N,Y] where Y is LAN on P, N is B’s bridge # w.r.t. (P1,P) –Update largest frame field – LF min= MF(P) –Recalculate CRC –Transmit on P

CEN 5501C - Computer Networks - Spring UF/CISE - Newman14 All Paths Explorer Frames If length = 6-28 bytes of route info, B forwards to every port P (to LAN Y) except P1 (from LAN X) iff –The final collected hop is X (else drop, log error) –Y is not yet in collected RI If port P is eligible, B does –Adjust RI += 2 –Replace [X] with [X|N,Y] in RI –Update largest frame field – LF min= MF(P) –Recalculate CRC –Transmit on P If RI field is full, B discards

CEN 5501C - Computer Networks - Spring UF/CISE - Newman15 Spanning Tree Explorer Frames Source host first sends with no route info Handled in same way as APEx Frame, except –Don’t check if output LAN repeated –Only forward if STEx Frame appears on a spanning tree port –Only forward to other spanning tree ports

CEN 5501C - Computer Networks - Spring UF/CISE - Newman16 Hybrids LANs and Bridges TB and SR LANs wish to interconnect SR-TB Bridges –TB frames –SR frames Pure SR and SR-TB declared “non- standard” … so 4 types of bridges SRT Bridges –TB frames treated as a TB –SR frames treated as an SR Bridge

CEN 5501C - Computer Networks - Spring UF/CISE - Newman17 End Systems No standard How to route – build cache When to find a route –Don’t keep trying if failed recently When to flush cache –Higher layer signals –No recent traffic from D on cached route –After some time, even if in use Be careful!

CEN 5501C - Computer Networks - Spring UF/CISE - Newman18 How to Find a Route Would be nice if source could distinguish –Destination down –Destination only available via transparent bridging –D reachable by SR, but SR worse than TB route –D reachable by SR, and SR better than TB route Strategy 1 –S sends APEx frame to D –D replies with SR frame for each APEx frame, with direction flipped –D does not modify its cache Strategy 2 –S sends “need route” frame transparently first –D replies with “need route response” transparently –S then tries to find SR –Need standard!

CEN 5501C - Computer Networks - Spring UF/CISE - Newman19 How to Find a Route Strategy 3 –S sends “need route” frame transparently first –D replies with APEx –S picks a route from APEx frames Strategy 4 –Like 3, but D replies with two frames: APEx frame Transparent “need route reply” Strategy 5 –S sends APEx frame to D –D selects a single route, replies with SR frame for that route, with direction flipped –S updates its cache

CEN 5501C - Computer Networks - Spring UF/CISE - Newman20 How to Find a Route Strategy 6 –Always use transparent bridging!

CEN 5501C - Computer Networks - Spring UF/CISE - Newman21 How to Find a Route Strategy 6 –Always use transparent bridging! + Simple + Always works + No route discovery overhead –Won’t find a better SR route –End STAs don’t find maximum frame size –Traffic only uses spanning tree paths

CEN 5501C - Computer Networks - Spring UF/CISE - Newman22 Destination Route Discovery D can obtain information from APEx –Route selected may not be same as S’s –No assurance route works D can wait until it has to send frames –Same issues, less efficient! D can learn from received data frames –Symmetric –Must examine all data frames –What to do if route differs?

CEN 5501C - Computer Networks - Spring UF/CISE - Newman23 Route Selection End STA may have many options Criteria for route selection –First received APEx frame –Route with largest max frame size –Route with fewest hops –Combine the above –Most recently received route (  )

CEN 5501C - Computer Networks - Spring UF/CISE - Newman24 TB vs SR Overhead –Route discovery –RI field –Suboptimal routing (esp. TB) Configuration ease –LAN number assignment –Bridge number assignment Universality –End nodes must have compatible SR –TB does not find max frame size –Route change may change max frame size….