This is a list of different sources of information that I have used to get more information about different behaviours and so on. Some resources may have been left out on purpose while others where left out on mistake. If you have any well written resource, let me know and I may add it here.
RFC 791 - Internet Protocol - The original document specifying the IP protocol. This document was edited by J. Postel from the DoD Standard Internet Protocol.
RFC 792 - Internet Control Message Protocol - The definitive resource for all information about ICMP packets. Whatever technical information you need about the ICMP protocol, this is where you should turn first. Written by J. Postel.
RFC 793 - Transmission Control Protocol - This is the original resource on how TCP should behave on all hosts. This document has been the standard on how TCP should work since 1981 and forward. Extremely technical, but a must read for anyone who wants to learn TCP in every detail. This was originally a Department of Defense standard written by J. Postel.
RFC 959 - File Transfer Protocol - This is the RFC documenting the FTP protocol and how it works. Originally written by J. Postel and J. Reynolds.
RFC 1072 - TCP Extensions for Long-Delay Paths - This RFC discusses possible solutions to specific problems in paths or streams that have extremely long round-trip delay or extremely high bandwidth and what can be done about TCP sequence number reusing among other things. This RFC was obsoleted by RFC 1323, but is available here since it contains valuable information. This RFC was written by V. Jacobson and R. Braden.
RFC 1122 - Requirements for Internet Hosts -- Communication Layers - This RFC specifies requirements for hosts that are working on the Internet. Edited by R. Braden and written by IETF.
RFC 1123 - Requirements for Internet Hosts -- Application and Support - This document describes the requirements for all internet hosts regarding the Application layer protocol, and all of their supporting protocols and layers. It was edited by R. Braden
RFC 1323 - TCP Extensions for High Performance - This RFC obsoletes RFC 1072 previously described in this section. This RFC gives a brief explanation of possible problems with connections that have extremely long round-trip delays and/or extremely high bandwidth, as well as a set of solutions to this problem. This RFC was written by V. Jacobson, R. Braden and D. Borman.
RFC 1337 - TIME-WAIT Assassination Hazards in TCP - This RFC discusses a few possible failures that TCP connections may get into. It also discusses a few remedies to these problems, and identifies a simple fix that can be used. This RFC was written by R. Braden.
RFC 1812 - Requirements for IP Version 4 Routers - RFC 1812 discusses the requirements of all IPv4 routers. This is a must read if you are in any way serious about your routing, or if you need to implement anything that has to do with routing. This RFC is based on 1716, which it also obsoletes, and was updated by F. Baker.
RFC 2018 - TCP Selective Acknowledgement Options - This RFC discusses the insertion of a new TCP Option called SACK, or Selective Acknowledgement, which gives better possibilities to acknowledge parts of a TCP window. Written by M. Mathis, J. Mahdavi, S. Floyd and A. Romanow.
RFC 2883 - An Extension to Selective Acknowledgement (SACK) Option for TCP - This RFC extends RFC 2018 and defines how to acknowledge duplicate incoming packets and how to handle these duplicate packets. Written by S. Floyd, J. Mahdavi, M. Mathis and M. Podolsky.
RFC 2884 - Performance Evaluation of Explicit Congestion Notification (ECN) in IP Networks - This is a performance evaluation of ECN and how it works in IP networks. This is an informational RFC only, and contains no specific information on how ECN works. The RFC was written by J. Hadi Salim and U. Ahmed.
RFC 3168 - The Addition of Explicit Congestion Notification (ECN) to IP - This RFC defines how ECN is to be used on a technical level and how it should be implemented in the TCP and IP protocols. Written by K. Ramakrishnan, S. Floyd and D. Black.
Guide to IP Layer Network Administration with Linux - A large guide covering how IP works in Linux, and how to administrate it. It covers most tools, and common IP concepts that may arise. Especially look closer at the ARP-flux problems covered, which is a very good explanation.
ECN-under-Linux Unofficial Vendor Support Page - A great page which contains information about the ECN problems, and a brief list of the most common hardware that is problematic.
ECN: Executive Summary - A mail sent to the Linux kernel mailing list by Dax Kelson claiming 8% of the Internet is unreachable with ECN enabled. This mail is dated in 2000, so this has changed much by today.
TCP Tuning Guide - A document discussing different measures one may take to tune TCP networks for better performance. It was written by a project group lead by Brian L. Tierney, DIDC (Data Intensive Distributed Computing group) at Lawrence Berkeley Labs. Most of this information does not apply to networks slower than giga ethernet.
services.txt - This is an example services file, which can be found in /etc/services on most Unix systems. This services file was grabbed from a Slackware 8.0 system.
protocols.txt - This simple file is normally hosted in every unix machine as /etc/protocols. The protocols file contains name and numbers representing all the different protocols in the IP layer. This file was taken from a default Slackware 8.0 machine.
ip-sysctl.txt - This is the classical documentation for the ip-sysctl functions in the linux kernel. This document was taken out of the linux kernel 2.4.14 and has not evolved a lot in the last couple of months, as of writing this.
ip_dynaddr.txt - This file contains information about the IPv4 ip_dynaddr option available in sysctl. This file was distributed with the linux kernel 2.4.14 and was originally written by JuanJo Ciarlante.
ip-param.txt - This document contains information about IP Options mainly, and which RFC's to look in, to get more information about the specific IP options.
netdev mailinglist - The netdev mailinglist is the main mailing list for the network developers in Linux. This is the final stop if you find something you don't understand or don't know how it works.