Computer Networks

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1

Computer Networks

Chapter 4 – Source Routing Bridges


Why Source Routing?

• It exists– 802 standards– IP option– GWEN– MANETs

• Instructive alternative– Variety provides insight– Variety provides options


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?

DataDest Addr Src Addr


Routing Information (RI)• Magic bit

– Multicast bit in source address field

(never send from a multicast address!)

DataDest Addr Src Addr

RIDest Addr Src Addr Data

Multicast bit = 0

Multicast bit = 1

Normal packet (transparent bridge routing):

Source routed packet with Routing Information field:


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)


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


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?


Bridge Numbers

• Suppose there were multiple layers…

LAN A LAN C

… now how many copies show up on LAN C?

LAN B


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 (N2 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”


Why Parallel Bridges?

• Robustness• Underpowered bridges• Low capacity links

– Need SR to use both– Best if both links between

same bridges

LAN A LAN B

LAN A LAN B

LAN A LAN B


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


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


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


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


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


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


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!


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!


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



– Always use transparent bridging!



– 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


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?


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 ()


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….

Computer Networks

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