Exit Zen
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../../protocols/ipv6.rst:570
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If ``A`` sends a 1500 bytes packet, ``R1`` will return an ICMPv6 error message indicating a maximum packet length of 1400 bytes. ``A`` would then fragment the packet before retransmitting it. The small fragment would go through, but the large fragment will be refused by ``R2`` that would return an ICMPv6 error message. ``A`` can fragment again the packet and send it to the final destination as two fragments.
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../../protocols/wifi.rst:105
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The astute reader may have noticed that the 802.11 data frames contain three 48-bits address fields [#f4addresses]_ . This is surprising compared to other protocols in the network and datalink layers whose headers only contain a source and a destination address. The need for a third address in the 802.11 header comes from the `infrastructure` networks. In such a network, frames are usually exchanged between routers and servers attached to the LAN and WiFi devices attached to one of the access points. The role of the three address fields is specified by bit flags in the `Frame Control` field.
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../../protocols/tcp.rst:558
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by extension, some implementations respond with an `RST` segment to a segment that is received on an existing connection but with an invalid header :rfc:`3360`. This causes the corresponding connection to be closed and has caused security attacks :rfc:`4953`
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../../glossary.rst:203
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The Secure Shell (SSH) Transport Layer Protocol is defined in :rfc:`4253`
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../../principles/linkstate.rst:19
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Usually, the same weight is associated to the two directed edges that correspond to a physical link (i.e. :math:`R1 \rightarrow R2` and :math:`R2 \rightarrow R1`). However, nothing in the link state protocols requires this. For example, if the weight is set in function of the link bandwidth, then an asymmetric ADSL link could have a different weight for the upstream and downstream directions. Other variants are possible. Some networks use optimization algorithms to find the best set of weights to minimize congestion inside the network for a given traffic demand [FRT2002]_.
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../../principles/reliability.rst:712
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Sliding window example
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../../principles/sharing.rst:780
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The simplest form of feedback that the network can send to a source is a binary feedback (the network is congested or not congested). In this case, a `linear` rate adaptation algorithm can be expressed as :
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../../protocols/email.rst:272 • ../../protocols/email.rst:291 • ../../protocols/email.rst:305
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8-bit
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../../protocols/ipv6.rst:572
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In practice, an IPv6 implementation does not store the transmitted packets to be able to retransmit them if needed. However, since TCP (and SCTP) buffer the segments that they transmit, a similar approach can be used in transport protocols to detect the largest MTU on a path towards a given destination. This technique is called PathMTU Discovery :rfc:`1981`.
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../../protocols/wifi.rst:107
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When a frame is sent from a WiFi device to a server attached to the same LAN as the access point, the first address of the frame is set to the MAC address of the access point, the second address is set to the MAC address of the source WiFi device and the third address is the address of the final destination on the LAN. When the server replies, it sends an Ethernet frame whose source address is its MAC address and the destination address is the MAC address of the WiFi device. This frame is captured by the access point that converts the Ethernet header into an 802.11 frame header. The 802.11 frame sent by the access point contains three addresses : the first address is the MAC address of the destination WiFi device, the second address is the MAC address of the access point and the third address the MAC address of the server that sent the frame.
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../../protocols/tcp.rst:559
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by extension, some implementations send an `RST` segment when they need to close an existing TCP connection (e.g., because there are not enough resources to support this connection or because the remote host is considered to be unreachable). Measurements have shown that this usage of TCP `RST` is widespread [AW05]_
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../../glossary.rst:301
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standard query
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../../principles/linkstate.rst:24
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When a link-state router boots, it first needs to discover to which routers it is directly connected. For this, each router sends a HELLO message every `N` seconds on all its interfaces. This message contains the router's address. Each router has a unique address. As its neighboring routers also send HELLO messages, the router automatically discovers to which neighbors it is connected. These HELLO messages are only sent to neighbors that are directly connected to a router, and a router never forwards the HELLO messages that it receives. HELLO messages are also used to detect link and router failures. A link is considered to have failed if no HELLO message has been received from a neighboring router for a period of :math:`k \times N` seconds.
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../../principles/reliability.rst:715
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In practice, as the frame header includes an `n` bits field to encode the sequence number, only the sequence numbers between :math:`0` and :math:`2^{n}-1` can be used. This implies that, during a long transfer, the same sequence number will be used for different frames and the sliding window will wrap. This is illustrated in the figure below assuming that `2` bits are used to encode the sequence number in the frame header. Note that upon reception of `OK1`, the sender slides its window and can use sequence number `0` again.
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../../principles/sharing.rst:782
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:math:`rate(t+1)=\alpha_C + \beta_C rate(t)` when the network is congested
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../../protocols/email.rst:272
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00010100 11111011 10011100 00000011 11011001 01111110
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../../protocols/ipv6.rst:576
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When a TCP segment is transported in an IP packet that is fragmented in the network, the loss of a single fragment forces TCP to retransmit the entire segment (and thus all the fragments). If TCP was able to send only packets that do not require fragmentation in the network, it could retransmit only the information that was lost in the network. In addition, IP reassembly causes several challenges at high speed as discussed in :rfc:`4963`. Using IP fragmentation to allow UDP applications to exchange large messages raises several security issues [KPS2003]_.
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../../protocols/wifi.rst:109
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802.11 control frames are simpler than data frames. They contain a `Frame Control`, a `Duration` field and one or two addresses. The acknowledgment frames are very small. They only contain the address of the destination of the acknowledgment. There is no source address and no `Sequence Control` field in the acknowledgment frames. This is because the acknowledgment frame can easily be associated to the previous frame that it acknowledges. Indeed, each unicast data frame contains a `Duration` field that is used to reserve the transmission channel to ensure that no collision will affect the acknowledgment frame. The `Sequence Control` field is mainly used by the receiver to remove duplicate frames. Duplicate frames are detected as follows. Each data frame contains a 12 bits sequence number in the `Sequence Control` field and the `Frame Control` field contains the `Retry` bit flag that is set when a frame is transmitted. Each 802.11 receiver stores the most recent sequence number received from each source address in frames whose `Retry` bit is reset. Upon reception of a frame with the `Retry` bit set, the receiver verifies its sequence number to determine whether it is a duplicated frame or not.
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../../protocols/tcp.rst:561
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When an `RST` segment is sent by a TCP entity, it should contain the current value of the `sequence number` for the connection (or 0 if it does not belong to any existing connection) and the `acknowledgment number` should be set to the next expected in-sequence `sequence number` on this connection.
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../../glossary.rst:303
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For DNS servers and resolvers, a standard query is a query for a `A` or a `AAAA` record. Such a query typically returns an IP address.
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