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Diffstat (limited to 'doc/HACKING')
| -rw-r--r-- | doc/HACKING | 64 |
1 files changed, 2 insertions, 62 deletions
diff --git a/doc/HACKING b/doc/HACKING index 504d6d142..969ac690c 100644 --- a/doc/HACKING +++ b/doc/HACKING @@ -574,67 +574,7 @@ The pieces. Streams are multiplexed over circuits. Cells. Some connections, specifically OR and OP connections, speak - "cells". This means that data over that connection is bundled into 256 - byte packets (8 bytes of header and 248 bytes of payload). Each cell has + "cells". This means that data over that connection is bundled into 512 + byte packets (14 bytes of header and 498 bytes of payload). Each cell has a type, or "command", which indicates what it's for. -Robustness features. - -[XXX no longer up to date] - Bandwidth throttling. Each cell-speaking connection has a maximum - bandwidth it can use, as specified in the routers.or file. Bandwidth - throttling can occur on both the sender side and the receiving side. If - the LinkPadding option is on, the sending side sends cells at regularly - spaced intervals (e.g., a connection with a bandwidth of 25600B/s would - queue a cell every 10ms). The receiving side protects against misbehaving - servers that send cells more frequently, by using a simple token bucket: - - Each connection has a token bucket with a specified capacity. Tokens are - added to the bucket each second (when the bucket is full, new tokens - are discarded.) Each token represents permission to receive one byte - from the network --- to receive a byte, the connection must remove a - token from the bucket. Thus if the bucket is empty, that connection must - wait until more tokens arrive. The number of tokens we add enforces a - longterm average rate of incoming bytes, yet we still permit short-term - bursts above the allowed bandwidth. Currently bucket sizes are set to - ten seconds worth of traffic. - - The bandwidth throttling uses TCP to push back when we stop reading. - We extend it with token buckets to allow more flexibility for traffic - bursts. - - Data congestion control. Even with the above bandwidth throttling, - we still need to worry about congestion, either accidental or intentional. - If a lot of people make circuits into same node, and they all come out - through the same connection, then that connection may become saturated - (be unable to send out data cells as quickly as it wants to). An adversary - can make a 'put' request through the onion routing network to a webserver - he owns, and then refuse to read any of the bytes at the webserver end - of the circuit. These bottlenecks can propagate back through the entire - network, mucking up everything. - - (See the tor-spec.txt document for details of how congestion control - works.) - - In practice, all the nodes in the circuit maintain a receive window - close to maximum except the exit node, which stays around 0, periodically - receiving a sendme and reading more data cells from the webserver. - In this way we can use pretty much all of the available bandwidth for - data, but gracefully back off when faced with multiple circuits (a new - sendme arrives only after some cells have traversed the entire network), - stalled network connections, or attacks. - - We don't need to reimplement full tcp windows, with sequence numbers, - the ability to drop cells when we're full etc, because the tcp streams - already guarantee in-order delivery of each cell. Rather than trying - to build some sort of tcp-on-tcp scheme, we implement this minimal data - congestion control; so far it's enough. - - Router twins. In many cases when we ask for a router with a given - address and port, we really mean a router who knows a given key. Router - twins are two or more routers that share the same private key. We thus - give routers extra flexibility in choosing the next hop in the circuit: if - some of the twins are down or slow, it can choose the more available ones. - - Currently the code tries for the primary router first, and if it's down, - chooses the first available twin. |