Internet: Best Effort. L11: Protocols and Network layer. Protocol. End hosts implement everything else

Transcription

1 L: Protocols and Network layer Frans Kaashoek 6.0 Spring 0 Some slides are from lectures by Nick Mckeown, Ion Stoica,ina Katabi, Hari alakrishnan, Sam Madden, and Robert Morris Internet: est ffort No Guarantees: Variable elay (jitter) Variable rate Packet loss uplicates Reordering Maximum length nd hosts implement everything else Protocol SMTP TP mail addresses,, c, etc. Reliable, flow-controlled connection SMTP TP efines the structure of a conversation Typical a sequence of messages, each with its own header xamples: HP, NS, UP, SMTP, TP, IP, Internet protocols defined in text documents (RFs) IP: best-effort vers HLen TOS tal Length Hop count TTL I Protocol Flags FRAG Offset checksum SR IP Address IP Address (OPTIONS) (PA)

2 Layering of protocols ach layer adds/strips off its own header ach layer may split up higher-level data ach layer multiplexes multiple higher layers ach layer is (mostly) transparent to higher layers The Internet Stack Protocol Stack App ata Hdr Transport TP / UP TP packet Network IP ata Hdr IP packet mail TP data TP IP thernet IP WIFI IP thernet App Transport Network The Internet Hour glass mail Web VoIP PP RTSP TP UP IMP IP ther Sonet ATM Fiber TP AT5 WiFi GSM "verything over IP, and IP over everything" Layer Problem: eliver data from one end of the link to the other Need to address (6.0): its Analog its Framing rrors Medium Access ontrol

3 Forwarding Network Layer: finds a path to the destination and forwards packets along that path ifference between routing and forwarding Routing is finding the path Forwarding is the action of sending the packet to the next-hop toward its destination ach router has a forwarding table Forwarding tables are created by a routing protocol R Forwarding table at R st. Addr A R R R A Inside a router The IP Header Forwarding Table vers HLen TOS tal Length, ingress Forwarding ecision, egress Hop count TTL I Protocol Flags FRAG Offset checksum, ingress, ingress hoose gress hoose gress, egress, egress SR IP Address IP Address (OPTIONS) (PA) 4, ingress hoose gress 4, egress

4 And switches today Forwarding an IP Packet Alcatel 7670 RSP Lookup packet s in forwarding table If known, find the corresponding outgoing link If unknown, drop packet ecrement TTL and drop packet if TTL is zero; update header hecksum Forward packet to outgoing port Transmit packet onto link Avici TSR isco GSR 46 6ft x ft x.5ft 4. kw power 60 Gb/s cap. TX8 Juniper TX8/T640 Lucent 5SS telephone switch The Routing Problem: Generate forwarding tables A Goals: No loops, short paths, etc. 4

5 Vector Routing Protocol Initialization ach node knows the path to itself For example, initializes its paths nd layer A 5

6 Vector Step : Advertisement ach node tells its neighbors its path to each node in the graph A Vector Step : Update Route Info ach node use the advertisements to update its paths received: From A: A From : From : For example, receives: From A: From : A From : updates its paths: nd layer nd layer A <A> <> <> Note: At the end of first round, each node has learned all one-hop paths Vector Periodically repeat Steps & In round, receives: From A: updates its paths: nd layer A <A> <> <> From : A <> <> <> <> From : <> <> nd layer A <A> <> <> <, > Questions About Vector How do we avoid permanent loops? What happens when a node hears multiple paths to the same destination? What happens if the graph changes? Note: At the end of round, each node has learned all two-hop paths 6

7 Questions About Vector How do we ensure no loops? When a node updates its paths, it never accepts a path that has itself What happens when a node hears multiple paths to the same destination? It picks the better path (e.g., the shorter number of hops) What happens if the graph changes? Algorithm deals well with new links deal with links that go down, each router should discard any path that a neighbor stops advertising domain- Hierarchical Routing domain- domain- Interior router order router Internet: collection of domains/networks Inside a domain: Route over a graph of routers etween domains: Route over a graph of domains Address consists of omain Id, Node Id Advantage Scalable Smaller tables Smaller messages elegation ach domain can run its own routing protocol Hierarchical Routing isadvantage Mobility is difficult Address depends on geographic location Sup-optimal paths.g., in the figure, the shortest path between the two machines should traverse the yellow domain. Routing: many open issues Misconfigurations between domains? Flat addresses and scalable? Routing in multihop WiFi networks? Routing in peer-to-peer networks? 7

8 Summary Protocols Layering of protocols Network layer: forwarding & Routing -vector routing protocol 8