# Spring 2006CS 685 Network Algorithmics1 Principles in Practice CS 685 Network Algorithmics Spring 2006.

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Spring 2006CS 685 Network Algorithmics1 Principles in Practice CS 685 Network Algorithmics Spring 2006

CS 685 Network Algorithmics2 Scheduling for ATM VC's scheduler transmit Background: Asynchronous Transfer Mode: virtual-circuit service Individual circuits may be flow- controlled for various reasons –bandwidth allocation –end-to-end flow control At each output port, each v.c. has its own queue of packets waiting for transmission A scheduler selects which packet to transmit next, according to per-v.c. state

Spring 2006CS 685 Network Algorithmics3 Scheduling for ATM VC's scheduler transmit Background: To be eligible for selection a v.c. must have: –at least one "credit" (i.e. it has not yet used up its allocation) –at least one packet queued "Credits" may be accumulated by: –passage of time (for BW allocation) –transmission from upstream (for end-to-end flow control)

Spring 2006CS 685 Network Algorithmics4 Scheduling for ATM VC's scheduler transmit Problem: Too expensive to find next ready queue via linear search, if there are 1000's of virtual circuits How can we avoid spending time looking at many queues that are not ready? Hint: P12 "add state for speed"

Spring 2006CS 685 Network Algorithmics5 Scheduling for ATM VC's scheduler Solution (version 0): Maintain a queue of ready v.c.'s When a flow becomes ready, add it via tail pointer –Flow becomes ready when #pkts > 0, #credits 0  1 #credits > 0, #pkts 0  1 When a flow is serviced (i.e. a packet is transmitted), remove it via head pointer –If still ready after xmission, place it back in queue via tail Head Tail

Spring 2006CS 685 Network Algorithmics6 Scheduling for ATM VC's scheduler Problem: Credits and packets arrive asynchronously How to ensure v.c. is not added to the list multiple times? Ensure that events that may cause transition between "ready" states are "atomic" –But locking is expensive! Head Tail

Spring 2006CS 685 Network Algorithmics7 Scheduling for ATM VC's scheduler Solution 1: Add a ready/not ready bit to each v.c.'s state: –bit set means the v.c. is already in the queue –clear bit before adding to/removing from the queue Head Tail

Spring 2006CS 685 Network Algorithmics8 Scheduling for ATM VC's scheduler Problem: Extend the scheme to allow some v.c.'s to get "more opportunities to send", based on administratively- assigned "weights" –E.g. a v.c.'s with a weight of 2 would get twice as many opportunities as a v.c. with a weight of 1 Possible Solution: Multiply credits by weight when they arrive Head Tail

Spring 2006CS 685 Network Algorithmics9 Passive Monitor Using Bridge Hardware Background: Ethernet bridge: box with connections to multiple Ethernets Keeps track of which interface a 48-bit MAC address lives on Forwards incoming packets out the right i/f for the destination address Contains special "CAM" hardware: –Keeps track of up to 64000 (addr, i/f #) pairs –Given addr, returns i/f # in just 1.4  sec

Spring 2006CS 685 Network Algorithmics10 Bridge Hardware Lookup 20:AB:0C:71:6F:A8I/F 4 02:00:20:6A:F3:19I/F 1 up to 64000 entries Address: 00:06:20:12:CD:BB Address: 00:06:20:12:CD:BB  I/F 6 00:06:20:12:CD:BBI/F 6

Spring 2006CS 685 Network Algorithmics11 Passive Monitor Using Bridge Hardware Problem: Want to build a monitor to count the number of packets sent between source, destination MAC address pairs Keep a counter per (S,D) pair, where S, D are 48-bit MAC addresses Need to exploit the existing bridge hardware (P4c) to do the lookup in less than 64  sec (minimum inter-packet time on 10Mbps Ethernet) There are about 1000 possible sources and 1000 destinations, but only around 20000 pairs active at any time

Spring 2006CS 685 Network Algorithmics12 Passive Monitor Using Bridge Hardware Naive Solution: Map each 48-bit address to a smaller one (say 10 bits) using one lookup each Store 2-D array indexed by 10-bit quantities Bad: array size 1000000 Better: Map each 48-bit address to a smaller one (say 24 bits) using one lookup each Concatenate 24-bit IDs to get a single 48-bit ID for the connection Retrieve counter using another hardware lookup with that ID

Spring 2006CS 685 Network Algorithmics13 Passive Monitor Solution Src = 12:34:56:78:9A:BC SrcID = 99:98:1B Dest = 12:34:56:78:9A:BC DestID = 01:22:FE CounterID = 99:98:1B:01:22:FE Counter = 322

Spring 2006CS 685 Network Algorithmics14 Refinement I Set of active source-destination pairs changes over time. How to handle this without storing state for every pair that ever communicated since power-on? Solution: –Run a "sweeper" process in the background, which does the following: Mark every counter periodically –Mark is cleared when counter is incremented Once per period, dump marked counters to secondary storage and reclaim them (make them available or use with new pairs)

Spring 2006CS 685 Network Algorithmics15 Challenge Can we solve the problem using only two lookups with bridge hardware, without requiring extra memory?

Spring 2006CS 685 Network Algorithmics16 Tries With Node Compression Background: Trie: tree data structure –Each node is an array of M = 2 c elements (M < 32) –Each entry is either a key (value) or a pointer to another trie node, or empty –Empty space is wasted Key: X Key: Q Key: X 5 out of 24 entries used  apply P1

Spring 2006CS 685 Network Algorithmics17 Tries With Node Compression To search the trie: –Input string broken into c-bit "chunks" –First chunk used as index into root node's array –If the indexed entry is: null: search terminates (not found) Key: search terminates (found, result = Key) Pointer to another node: search continues there with next chunk Key: X Key: Q Key: X

Spring 2006CS 685 Network Algorithmics18 Tries With Node Compression Problem: how to compress the tree to remove wasted space without slowing search more than a small factor? Can we replace the node array with a linked list? Not good: takes up to factor of M longer to search Key: X Key: Q Key: X

Spring 2006CS 685 Network Algorithmics19 Tries With Node Compression Idea: –Use a bitmap (P14) to indicate which entries are missing –Replace M-element array with array containing only non-null entries –To search with c-bit index k (indices start at 1): Convert c-bit index k to smaller index –If bit k of bitmap is 0  0, else  #of 1's in first k bits Key: X Key: Q Key: X 10010010 00010000 00000100 Original index: 4 New index: 2

Spring 2006CS 685 Network Algorithmics20 Tries With Node Compression Efficiency: –Can count # 1's in bitmap using special hardware (or s/w) –Requires two memory accesses: one for bitmap one to read array entry –Slowdown: factor of 2+ Key: X Key: Q Key: X 10010010 00010000 00000100 Original index: 4 New index: 2

Spring 2006CS 685 Network Algorithmics21 Challenge I Challenge: Can we use table lookup to speed up counting # of bits set in bitmap in software (P2a, P14)? Solution (for M=8, c=3): –There are 256 possible bitmap values –Keep a table indexed by [bitmap value, position], containing the (precomputed) number of 1 bits up to that position (3 bits per table entry) –Total: 256 x 8 x 3 = 6144 bits  probably doable in hardware... 00000000 00000000 01002300 01001100...

Spring 2006CS 685 Network Algorithmics22 Challenge II Challenge: What if the bitmap is really huge? (e.g. c=16) –Table in previous solution would have 2 64K rows! How can we speed up counting bits for such a large bitmap, with at most one additional memory access? Solution: Apply P12 "add state for speed" and P12a "compute incrementally" –Logically divide bitmap into chunks (say 32 bits each) –For each node, keep an array B with one element per chunk For 64K bitmap in 32-bit chunks: B has 2K elements of 16 bits each = 1K 32-bit words (still much smaller than 64K entries!) B[j] contains # of 1 bits in positions 1 through 32(j-1) To count 1's up through position k: B[k] + #1's in chunk k –Count 1's in chunk k individually

Spring 2006CS 685 Network Algorithmics23 Packet Classification Background: TCP/IP "flows" identified by some set of fields in packet headers, including: –Source, destination IP address –Source, destination port number –Protocol (e.g. TCP or UDP) –Higher-level (application) information Packet "filter" = specification of fields that define a set of packets "of interest" to some receiver –Example (simple): Source IP = X, Dest IP = * Protocol = UDP Source port = *, Dest port = Z

Spring 2006CS 685 Network Algorithmics24 Packet Classification Background: Consider a "router" that serves a group of receivers –Receivers specify packets they want to receive by giving filters – Router takes each incoming packet, matches it against all filters, and forwards to each receiver that has some matching filter Example: –To receive NBC's video transmission from Winter Olympics: specify source address of NBC host in Turin, UDP protocol, and a specific destination port

Spring 2006CS 685 Network Algorithmics25 Packet Classification Problem: How to compare each packet's headers against many (possibly thousands) of filter specifications? Observation: Caching (P11a) is not a very good solution –Caching is good for storing pairs of the form (x,  (x)) in order to avoid re-computing (or looking up)  (x) Store is keyed by x, which is typically small (48 bits or less) –Here the key could be How could we solve this if header contained a "flow identifier" field F in the network layer header? Note: IPv6 has such a field

Spring 2006CS 685 Network Algorithmics26 Packet Classification Problem: How to assign/use "flow ID" field? Solution: Let the sender assign it! –E.g. keep a local counter, increment for each flow Keep (in addition to the regular list of filters) a "fast" mapping from (source address, flow ID) to sets of receivers When a packet arrives, first search the "fast" list –If present, dispatch to indicated set of receivers –Otherwise search the "slow" list to determine set G of receivers –Then add (source address, flow ID, G) to fast list

Spring 2006CS 685 Network Algorithmics27 Classification: Tricky Bits If a source crashes and restarts flows with different flow IDs, chaos could result –Solution: Time out "Fast list" entries periodically Ensure rebooting host does not re-use old entries (e.g. start with last used flowID + 1) Each "slow list" entry has a pointer to its "fast list" entry When a packet fails to match any "fast list" entry, but does match a "slow list" entry, existing fast list filter should be invalidated When a receiver adds a new filter, "fast list" entries may need to be updated –How to do this?

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