Computer Networking - Fine-Grained Syllabus

Questions and Answers from the lecturer

Q. DHCP appears as a Tier 1 glossary keyword, but in the lecture slides it is discussed under SLAAC, which is marked as non-examinable this year. It also seems to receive relatively little attention in the supervision worksheet. Could you clarify whether DHCP itself is examinable?
A. The core ideas of DHCP are examinable: All that was taught is that it's a system for dynamic allocation of IP addresses out of a pool in preference to manual allocation techniques. It works on a broadcast subnet (e.g. a LAN) where a machine (MAC address) is typically given the same address as it had previously, if within a week or two's timeout. SLAAC was not mentioned in 25/26 by myself.

Q. Question 22 in the updated supervision worksheet refers to the four timers in TCP ARQ and the silly window syndrome, but these do not seem to appear in the lecture notes. Should students be expected to know these topics?
A. This was not lectured this year (25/26) and so that example sheet question is not relevant for Tripos.

Q. Questions 23(a)(ii) and 26 mention gateways and firewall functionality, which were only briefly touched on in lectures. Could you clarify the expected depth of understanding for gateways, firewalls, and VPNs?
A. Gateways were not really mentioned, except to comment on the standard meaning as a per-application middlebox that operates above the network layer and which typically has different network layers either side. The earlier use of the term in the Internet was as a 'border gateway router' between autonomous domains (IP on both sides), but in academic terminology, this is just a router with a different system for managing its tables.
VPN, VLAN, NAT and Firewall are Examinable. They were mentioned lightly in several lectures, but all that was taught can be summarised in four sentences. Firewalls and NAT boxes are middleboxes with the same network layer each side (e.g. IP) and are commonly bundled together. NAT translates between local IP addresses (e.g. 10.10.X.X or 192.128.X.X) and one or a small number of public IP addresses utilising the port address space and Firewalls refuse to support connections according to security rules they have (e.g. no incoming TCP connections at all). A VPN typically involves tunnelling and similar layer order violations to provide the illusion of being on a local LAN or subnet when actually positioned elsewhere in the country or world. A VLAN system maps several virtual LANs over one physical LAN and is typically used for enhanced security, being cheaper and more convenient than 'air gap' separation provided by multiple physical network connections, but hubs that support VLANs might have a small price premium and they certainly need configuration.

Q. Question 24(a) mentions both iterative and recursive DNS resolution, while the lecture notes mainly provide an iterative example. Should students be familiar with both forms of DNS resolution?
A. Differences of that nature were not lectured clearly by myself in 25/26. I did not give much detail at all. Rather than familiarity with specific resolution patterns used in the real world, I think it is better if candidates 1/ understand the purpose of the various basic DNS record forms lectured (MX, CNAME etc.), 2/ understand that the core principle is an hierarchic approach of caches and delegated authorities (with numerous ad-hoc local edits and exceptions) and 3/ are able to envision their own set of feasible use patterns, taking into account that the anycast notion can be used and abused by filling in routing tables to direct enquiries to a local or ISP-preferred DNS server.

Q DNSSEC does not appear to have been lectured this year, but it appears in the supervision worksheet and is not marked as non-examinable. Could you clarify whether it is within scope?
A. This was not lectured this year (25/26) and so that example sheet question is not relevant for Tripos.

Topic-by-Topic: Fine-grained Syllabus

Syllabus variations. The official syllabus (on the first tab) is slightly out-of-date w.r.t. what is lectured at the moment: the ordering of materials has varied since the syllabus was last updated.

A concise/unified list of the topics covered per lecture (and accordingly can be expected in Tripos 25/26):

In essence, everything that was lectured is examinable. Any of the slides in the PDF that were skipped in lectures are already marked with 'not examinable/lectured in 25/26' in red.

Candidates should also read the sections of the two textbooks that are specifically mentioned or linked to per-topic below.

Topic 1 (Foundations) subjects covered in 25/26 are:
Most aspects of this topic are built on subsequently so should not need listing here, but here they are anyway:
Spatial sharing/diversity (aka SDM but not a common acronym). TDM, FDM, WDM.
Simplex, duplex, half-duplex, unicast, multicast, broadcast, anycast.
Circuit vs packet switching. Connection oriented vs connectionless.
Mean and peak flow rate, busrtyness, statistical multiplexing gain.
Speed of light, round-trip time (RTT), bandwidth delay product, utilisation and queuing delay.
Not covered in 25/26: information flux is a term I would have added --- units are bits per second per unit area.

Topic 2 (Architecture and Philosophy) subjects covered in 25/26 are:
Modularity, layering and abstractions. OSI reference model details. When layering could be harmful.
First mention of VLANs, tunnelling.
Internet approach and IP layer narrow waist.
End-to-end vs middleboxes.
Not covered in 25/26: 'Only if sufficient' principle. Reliable file transfer example.

Topic 3a (Physical Layer) subjects covered in 25/26 are:
Note: nearly all of the detail under the Topic 3a heading is not relevant for a Computer Science degree. Instead, it is Computer Engineering.
The formal syllabus is on the first tab of the course web page; it does not explicitly mention the physical layer (L1).
But, candidates should understand the basic information capacity formula and the differences between cable and wireless, and the names and differentiating aspects of each of the topics presented, but need not study any of the block diagrams or power budgets presented. But aspects of the physical layer, especially wireless networks, inevitably affect the data link layer (Topic 3b).
Specifically: the need and role of each of the following aspects of the PHY interface to DLL should be more than vaguely understood: Tx clock, Tx data, Tx frame start signal, Tx write gate (or power level), Rx data, Rx clock, Rx frame signal, Rx RSSI received signal strength or carrier detect, other Rx metrics (such as coding violations or FEC rate experience).
An important aspect of PHYs is whether they are duplex or half-duplex, where the duplex form arises from a pair of simplex channels.
Another important aspect of some PHYs is the ability to support multiple channels (eg. CDMA or FDM).
[Error detection and correction is on the syllabus so has been moved from 3a to 3b for clarity.]

Topic 3b (Data-link Layer) subjects covered in 25/26 are:
Packet framing and MAC addressing.
Error detection and correction using CRC and checksums (with no details of GF2 arithmetic). Brief mention of hamming-distance based error correction. Ditto erasure channels.
Shared medium, media access control protocols: turn-taking, polling, contention.
Simple Aloha, slotted Aloha, CSMA, CSMA/CD. Contention window and scaling.
Radio media: hidden and exposed terminals. CTS/RTS collision avoidance (WiFi) basis.
Migration to switched L2. Spanning tree.
Virtual LAN key concepts - same as multiple physical NICs. VPN layer violations (for MAC broadcast and for firewall issues etc.).
Not covered in 25/26: HomePlug. Bluetooth. Data centre networks. I would have added an overview of Viterbi since an important case of layers hindering performance. Also AutoNET.

Topic 4 (Network Layer) subjects covered in 25/26 are:
naming, addressing, routing, switching/forwarding, router block diagrams, tradeoffs associated with positioning of buffers.
intra-autonomous domain routing protocols: Link State, DV/OSPF, response to disruptions, hierarchic vs flat, subnetting and longest-prefix match
delivery models, reliability dimensions/aspects.
NAT (with a brief mention of firewalls).
ICMP and traceroute.
Further protocols: ARP, DHCP. Also IPv6 (exceptionally briefly),
Not covered in 25/26: IP multicast, IP fragmentation, IPv6 (except first two slides, so no details of Neighbour Discovery or SLAAC).

Topic 5 (Transport Layer) subjects covered in 25/26 are:
Core functions: port multiplexing, reliability, end-to-end flow control, congestion avoidance.
Pros and cons of working over IP's narrow waist, perhaps with ECN.
End-to-end vs hop-by-hop considerations.
Connection-oriented transport protocols or sessions (briefly mentioned) a few times
Principles of reliable conveyance: bytestream, SR vs GBN,
Sliding windows: congestion and receiver window limits, AIMD, backoff/kickdown,
Measuring RTT and jitter.
Scaling: slow-start, rapid recovery, CUBIC,
Fairness definitions (one lectured only) and fair queuing.
TLS key points (no details lectured).
Not covered in 25/26: TCP establishment and teardown (3-way handshake), the 'TCP equation' numerical integration of the window, buffer sizing theory,

Topic 6 (Application Layer) subjects covered in 25/26 are:
Basic nature of P2P and client/server applications.
DNS, Resource Record 4-field structure and 4 types lectured. gethostbyname.
DNS load balancing via anycast, caching, soft state.
Basic structure of simple HTML (not lectured, but taken as common knowledge). HTTP 1.0 and 1.1.
Conditional GET.
Web caches/proxies, discussion about forward and reverse positioning. Akamia-style overlay network. Eager push of content.
CDN: Three distinctive approaches. Also multiple sites per server.
Evolution beyond HTTP 1.1 to 2,3,4 QUIC (numbering not important, but understanding the role of the various components, such avoiding HOL, avoiding round-trips, security,
JPEG/MPEG CBR/VBR basic familiarity assumed. Dynamic playout buffer sizing.
Not covered in 25/26: Secure DNS (DNSSEC), Cookies, dark web, P2P file sharing, email, DASH multimedia protocol and subsequent two or three slides to end of pack.

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