588 Section 5 Neil Spring May 4, 1999

Schedule Notes – (1 slide) Homework 2 –(9 slides) Multicast –(11 slides)

Notes Graded Homework Assignment 1 –Should have received it this afternoon –Generally applicable questions?? Homework 2 due Monday, May 10 Programming Assignment 2 due May 24 Final Project too!

Homework 2 Dealing with images (1 slide) Next few slides: each problem in a little detail Make sure you know what we’re asking for Give you the opportunity to ask questions on each problem individually

Homework 2: Problem 1c & 4 Image formats: EPS, PNG, GIF. Put your name on em, filename too. Visio should work well. Text descriptions ok too –topology = (A,B)(B,C) –timing = a -> b, wait t sec. OR, printed and mailed to: Neil Spring Box University of Washington Seattle, WA

Homework 2: Problem 1 Little extra detail because it comes up in –the multicast paper –your second programming assignment Routing Schemes –Link State –Distance Vector –Path-based distance vector Goal is to decide where to forward a packet

Problem 1: Link State Each router sends out an LSP describing what other routers it’s connected to Other routers can overlay LSPs to discover the complete map Challenges for link-state: –scalably distributing LSPs (reliable flooding) –scalably processing LSPs OSPF has additional features –load balancing

Problem 1: Distance Vector Each router sends a vector of distances to each destination Dynamic programming sort of solution: –I can reach my neighbor’s destinations in 1 extra hop (or better) Split horizon improvement –Don’t advertise routes back where they came from Challenges for distance vector: –instability after link failure

Path-based Distance Vector Think BGP (1991) List the (transit) AS’s packets would travel through –AS’s are stub, multihomed, or transit –Represented by a 16-bit number Should avoid loops BGP –supports policy based routing –focuses on getting there loop free

Homework 2: Problem 2 CIDR: Classless InterDomain Routing Classless –no “B”, “C” –just variable length prefixes Each internal node has degree 3. Pretty simple question

Homework 2: Problem 3 Stack of routers to visit along the path Each router can push a new (more explicit) route on to the stack to get to the next hop Is this better? –Stable? –Faster? –Less error prone? –Possible to implement? Justify your assertion!!!

Homework 2: Problem 4 Using tcpdump as a tool is fine –My slides from Week 4 might be helpful. Note that the timer isn’t correct. Presenting tcpdump output alone is not fine. Note that 20 data packets must be sent: yes, all 20 should be in the diagram Just a count of round trips in the third data packet drop part (with a little explanation) is fine

Homework 2: Problem 5 BW = MSS *C / ( RTT * sqrt(p)) Periodic packet loss important.

Multicast Multicast Routing in Datagram Internetworks and Extended LANs What to multicast? –Stuff you know some set of machines care about updates to shared state conferencing –Stuff you don’t know who cares about Service location

Why multicast? “Minimal” resources demands: –don’t have to unicast to everyone –don’t have to broadcast to those who don’t care Broadcast –can’t be filtered in hardware, –generally doesn’t make it very far into the network Senders and receivers can come and go; the network figures out how to move bits.

Groups Open: non-members can send to the group Closed: can’t Pervasive: there are members everywhere Sparse: low member to non-member ratio Local: few links

Refinement: Scope control Maybe want to try close destinations first. Avoids excessive responses Using TTL. If you know the only recipients you care about are close, don’t bother the Internet –File / Print Services –Parallel Computation

Single Spanning Tree Compute one spanning tree for all multicast Listen to see which attached segments care about that multicast When a packet arrives on an incident edge, and the other edge cares, forward. Have to maintain state about who cares. Not transparent (hosts have to keep caring)

Distance Vector Multicast Reverse Path –Flooding –Broadcasting –Truncated –Multicasting “there is potentially a different shortest-path tree from every sender to every multicast group”

Reverse Path Flooding Find a good tree Described in the RPB section of the book. Reverse-path –We know how to get from a source to a destination, so we presume a packet would come back the same way. Flooding –Forward those packets over all outgoing links

Reverse Path Broadcasting Avoid duplicates by electing a –router as the ‘parent’ of a –‘child’ LAN Only the elected parent can send the packet on the lan Typically the closest, using address to break ties.

Truncated RPB Routers on child links tell parents –the lan is not a leaf –somebody downstream would care Can prune uninterested leaves Single spanning tree already had this

Reverse Path Multicasting Don’t waste resources pruning unused multicast trees Prune on demand: –Non-membership report (NMR)

Link State instead? We’ve got a map of the network, and a new multicast packet shows up from a new source Dijkstra’s on demand Cache the result Might take a while to get the first packet through

Issues in multicasting Differences in receiver bandwidth Reliability –retransmission –redundant transmission Ordering/consistency of multiple senders