Layering and the network layer
CS168, Fall 2014 Sylvia Ratnasamy h p://inst.eecs.berkeley.edu/~cs168/fa14/ Today
‣ What is a network made of? ‣ How is it shared? ‣ How do we evaluate a network? ‣ How is communica on architected? ‣ BREAK ‣ The network layer
2 Internet Layers
Applica ons …built on… Reliable (or unreliable) transport …built on… L7 Application Best-effort global packet delivery L4 Transport …built on… L3 Network L2 Data link Best-effort local packet delivery L1 Physical …built on…
Physical transfer of bits Protocols and Layers
L7 Application Application L7
L4 Transport Transport L4
L3 Network Network L3 L2 Data link Data link L2 L1 Physical Physical L1
Communication between peer layers on different systems is defined by protocols Protocols at different layers
L7 Application SMTP HTTP DNS NTP
L4 Transport TCP UDP
L3 Network IP
L2 Data link Ethernet FDDI PPP
L1 Physical optical copper radio PSTN
There is just one network-layer protocol!
L7 Application Application L7
L4 Transport Transport L4
L3 Network Network L3 L2 Data link Data link L2 L1 Physical Physical L1
What gets implemented where? What gets implemented at the end systems? l Bits arrive on wire, must make it up to applica on l Therefore, all layers must exist at host! What gets implemented in the network? l Bits arrive on wire à physical layer (L1) l Packets must be delivered across links and local networks à datalink layer (L2) l Packets must be delivered between networks for global delivery à network layer (L3) l The network does not support reliable delivery l Transport layer (and above) not supported Simple Diagram l Lower three layers implemented everywhere l Top two layers implemented only at hosts
Applica on Applica on Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical End system Switch End system A closer look: end-system
l Applica on l Web server, browser, mail, game l Transport and network layer l typically part of the opera ng system l Datalink and physical layer l hardware/firmware/drivers What gets implemented in the network? l Bits arrive on wire à physical layer (L1) l Packets must be delivered across links and local networks à datalink layer (L2) l Packets must be delivered between networks for global delivery à network layer (L3) l Hence: l switches: implement physical and datalink layers (L1, L2) l routers: implement physical, datalink, network layers (L1, L2, L3) A closer look: network
link
switch
Internet Service Provider 1 2 A closer look: network
link
switch routers
switch
Internet Service Provider 1 3 Switches vs. Routers l Switches do what routers do but don’t par cipate in global delivery, just local delivery l switches only need to support L1, L2 l routers support L1-L3 l Won’t focus on the router/switch dis nc on l when I say switch, I almost always mean router l almost all boxes support network layer these days App msg msg
H Transport t msg Ht msg
H H H H Network n t msg n t msg Hn Ht msg
Link Hl Hn Ht msg Hl Hn Ht msg Hl Hn Ht msg
Physical
Alice Router Bob Logical Communica on l Layers interacts with peer’s corresponding layer
Applica on Applica on Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical Host A Router Host B Physical Communica on
l Communica on goes down to physical network l Then up to relevant layer
Applica on Applica on Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical Host A Router Host B A Protocol-Centric Diagram
host host
HTTP message HTTP HTTP
TCP segment TCP TCP
router router
IP packet IP packet IP packet IP IP IP IP
Ethernet Ethernet SONET SONET Ethernet Ethernet interface interface interface interface interface interface Why layers?
l Reduce complexity l Improve flexibility
19 Why not?
l sub-op mal performance
l cross-layer informa on o en useful
Ø several “layer viola ons” in prac ce
20 l What physical infrastructure is already available? l Reserve or on-demand? l Where’s my delay coming from? l What’s the right set of layers?
21 Backing up…
L7 Application SMTP HTTP DNS NTP
at ends L4 Transport TCP UDP
L3 Network IP In L2 Data link Ethernet FDDI networkPPP
L1 Physical optical copper radio PSTN
Why is this architecture good? Architectural Wisdom l Benefits of layering l reduce complexity, increase flexibility l “narrow waist” l simple, minimal requirements for interoperability l “smart ends, dumb network” l No applica on knowledge in network à more general l Minimal state in the network à more robust to failure Architectural Wisdom l David D. Clark (MIT) l Chief protocol architect for the Internet from 1981 l Co-authored two classics
l “End-to-End Arguments in System Design” (1981)
l “The Design Philosophy of the DARPA Internet Protocols” (1988) Example: Reliable File Transfer
Host A Host B
App App
OS OK OS
l Solu on 1: make each step reliable, and string them together to make reliable end-to-end process l Solu on 2: end-to-end check and retry Discussion l Solu on 1 is incomplete l What happens if any network element misbehaves? l Receiver has to do the check anyway! l Solu on 2 is complete l Full func onality can be en rely implemented at applica on layer with no need for reliability from lower layers End-to-end argument: Intui on l Some applica on requirements can only be correctly implemented end-to-end l reliability, security, etc. l Implemen ng these in the network is hard l every step along the way must be fail proof l End-systems l Can sa sfy the requirement without network’s help l Will/must do so, since they can’t rely on the network End-to-End Arguments in Clark’s words
“The func on in ques on can completely and correctly be implemented only with the knowledge and help of the applica on at the end points of the communica on system. Therefore, providing that func on as a feature of the communica on system itself is not possible. (Some mes an incomplete version of the func on provided by the communica on system may be useful as a performance enhancement.)” Recap
l Implemen ng func onality (e.g., reliability) in the network l Doesn’t reduce host implementa on complexity l Does increase network complexity l Decreases generality (of the network) l However, implemen ng func onality in the network can improve performance in some cases l e.g., consider a very lossy link Commonly used examples
l Error handling in file transfer l End-to-end, versus in-network encryp on l The par on of work between TCP and IP for reliable packet delivery l What about Quality of Service (QoS)? l Communica on throughput or delay guarantee Some consequences l In layered design, the E2E principle provides guidance on which layers are implemented where l “Dumb” network and “smart” end systems l O en credited as key to the Internet’s success l “Fate sharing” l Store state at the system en es that rely on the state l O en translated to keeping state out of routers Cracks in the E2E argument? l Ignores incen ves of different stakeholders l e.g., ISP looking for new revenue-genera ng services l e.g., users looking for a performance boost l Some mes we don’t trust the end-systems to do the job l e.g., firewalls, intrusion detec on systems
Recap: architectural decisions
l How to break system into modules? l Classic decomposi on into tasks l Where are modules implemented? l Hosts? Routers? Both? l Where is state stored? l Hosts? Routers? Both? Leads to three guiding principles
l How to break system into modules? l Layering l Where are modules implemented? l End-to-end principle l Where is state stored? l Fate-sharing BREAK!