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Session Directories and Multicast Address Allocation Mark Handley - - PowerPoint PPT Presentation
Session Directories and Multicast Address Allocation Mark Handley - - PowerPoint PPT Presentation
Session Directories and Multicast Address Allocation Mark Handley USC/ISI mjh@isi.edu A Little History The first multicast session directory was sd from Van Jacobson and Steve McCanne at LBNL in 1992. I wrote sdr in 1994, and eventually it
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Basic Address Allocation Idea
Sdr announces which multicast addresses are in use. To allocate an address, simply listen to know which addresses are used, choose randomly from those not in use, and announce the address you choose. Problem: Not all sessions you might clash with can be seen and so avoided. If I allocate a global scope address, it can clash with local TTL-scoped sessions elsewhere.
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TTL Scoping
Sender sets IP "time-to-live" field in packets. Each router decrements TTL. When TTL reaches zero, packet is dropped. TTL constrains how far a multicast packet can travel. Configured TTL thresholds in routers supplement the basic mechanism to divide the Mbone into appropriate scope-zones such as "Europe" or "site-local". When a session is created, the TTL to decide its scope is chosen. Announcement packets go to the same scope as the session they announce.
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Informed Partitioned Random Multicast Address Allocation (IPRMA)
To avoid clashes, split the address space into different ranges for each scope:
Probability
- f Allocation
Address Range ttl range 1-15 ttl range 127-255 ttl range 64-127 ttl range 47-63 ttl range 32-47 ttl range 15-31
- Prob. of
Allocation
- f an
address for a session with a ttl in the range 64-127 Address Range ttl range 64-127
224.2.80.0 224.2.100.255
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The Problem with IPRMA
Not all locations choose the same TTL thresholds: Within Europe, TTL 48 divides countries. In North America, TTL 48 thresholds are not used. How then do you partition the address space? Too many ranges result in many empty ranges and poor utilization. Too few ranges result in failures of the informed mechanism.
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Informed Mechanism Failure
100 1000 100 1000
Allocations before clash Address Space Size Multicast Addresses Allocated before Clash
R IR IPR 3 IPR 7 R ds1 R ds2 R ds3 R ds4 IR ds1 IR ds2 IR ds3 IR ds4 IPR 3-band ds1 IPR 3-band ds2 IPR 3-band ds3 IPR 3-band ds4 IPR 7-band ds1 IPR 7-band ds2 IPR 7-band ds3 IPR 7-band ds4
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Adaptive Address Space Partitioning
If we don’ t know in advance the TTL values used worldwide and the distribution of sessions between them, it makes sense for the partitioning to be adaptive: Grow a partition when many addresses are assigned from it. Shrink unused partitions. Van first proposed this, but the difficult part is doing it without having a partition at one site grow to overlap a different scope partition at another site. When this happens the "informed" part of the mechanism fails.
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Deterministic Adaptive Partitioning
To perform address allocation so that heterogeneity of distribution of sessions between partitions does not cause partition overlap: Allocate numerous partitions at the high-TTL end of the space Have occupied partitions "push" the lower-TTL partitions down the space. Have inter-partition gaps between occupied partitions to cope with short-term variations of allocation.
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Deterministic Adaptive Partitioning
probability
- f
allocation address range probability
- f
allocation address range probability
- f
allocation address range Bands with TTL less than t Band with TTL greater than or equal to t
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Simulations of Adaptive IPRMA
Adaptive IPRMA is designed to work well where the number of addresses allocated from a partition is to some extent stable. We can’ t simulate this by allocating until a clash occurs. New criterion: An address allocation scheme is acceptable if during the mean lifetime of a session, the probability of an address clash anywhere in the world is less than 50%.
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Simulations
Rough Algorithm
- 1. Allocate n sessions; reallocate until no clashes.
- 2. Delete one session.
- 3. Add one session.
- 4. Repeat from 2 until n sessions have been replaced.
Simulations without communities: New session is unrelated to old session. No stability in number of "local" groups Simulations with totally stable communities: New session has same TTL and originator as old session. Group distribution seen from each site fixed.
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Simulations: No local communities
100 1000 100 1000
Allocations before clash probability > 50% Address Space Size
AIPR-1 (20% gap) AIPR-2 (50% gap) AIPR-3 (60% gap) AIPR-4 (70% gap) AIPR-H (hybrid) IPR 3-band IPR 7-band
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Simulations: Stable communities
100 1000 100 1000
Allocations before clash probability > 50% Address Space Size AIPR-1 (20% gap) AIPR-2 (50% gap) IPR 3-band IPR 7-band
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Reality?
How can we simulate the locality of future use? We can’t we could only speculate. The two simulation sets give lower and upper bounds for performance. Both scale O(n). Static allocation slightly outperforms adaptive allocation if we know the number of active partitions and their boundaries and their are no communities. In practice we don’ t know this. Thus the adaptive algorithm works well given the constraints.
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Packet Loss and Propagation Delay
The previous simulation ignores propagation delay and packet loss. Assume: mean session length: 2 hours mean advance announcement: 2 hours mean end-to-end delay: 200ms mean packet loss rate: 2% each announcement resent every 10 mins Mean end-to-end delay ~(0.98*0.2)+(0.02*600) = 12 secs Therefore approximately 0.1% (12 secs/4 hours) of sessions are not visible at any time.
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Packet Loss and Propagation Delay
10 100 1000 10000 100000 1e+06 100 1000 10000 100000 1e+06 Addresses Allocated Address Space Size Addresses allocated before probability of a clash is 0.5 Upper Bound (y=x) Lower Bound (y=x^0.5) i=0.01m i=0.001m i=0.0001m i=0.00001m
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Packet Loss
Instead of periodically announcing a session with a fixed base rate, start with a high rate (e.g. 1 per second) and exponentially back off down to a fixed base rate. With global propagation delay, this brings us to approximately i=0.0005m Scales well to around 10000 addresses allocated in a partition. This is not good enough!
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Two Solutions
Introduce hierarchy to address allocation. Do distributed address-range allocation between domains on a long timescales to mitigate propagation delays and loss. Do distributed individual address allocation within a domain on short timescales from the address-ranges. Add scalable mechanisms to detect and correct clashes when they do occur.
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Detecting Clashes
We’ d like any address allocator that notices a clash to be able to report it if the originator doesn’ t. There may be millions of allocators. How do we avoid clash report implosion? SRM (Sigcomm ’ 94) uses random timers chosen uniformly from a range [d1:d2]. d1 and d2 are chosen based on measurements
- f propagation delay.
One multicast response suppresses others. What can we do with no estimate of the population and no delay matrix?
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Exponential Random Timers
Uniform random timers are too dependent on knowing the number of nodes trying to respond. If we use exponentially distributed random timers we can largely remove this dependence.
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Simulation: Uniform Random Timers
A: Shortest path tree, delay=~distance 200 800 3200 12800 51200 2.0e5 8.2e5 3.3e6 1.3e7 200 400 800 1600 3200 6400 12800 25600 51200 1 10 100 1000 D2 (ms) No of sites No of responses B: Shared tree, delay=~distance C: Shortest path tree, delay=distance+random D: Shared tree, delay=distance+random
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Simulation: Exponential Random Timers
Mean no of responses 200 400 800 1600 3200 640012800 25600 51200 D2 (ms) 200 400 800 1600 3200 6400 12800 25600 51200 No of sites 1 10 100 1000 No of responses
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Summary
We’ ve designed a scalable approach for adaptive partitioning of the address space to deal with allocating addresses to TTL scoped sessions. The real problem is propagation delay. Solutions: Increase speed of light. Use a hierarchy with the upper levels
- perating on different timescales.